This glorious adverb – or is it? – was originally a phrase. Let us defer to the OED:

Etymology: In its origin a phrase, on prep., and weg , way n.¹ and int.¹, i.e. on (his, one’s) way , ‘on’ (as in ‘move on’), and thus ‘from this (or that) place.’ Already in Old English reduced to a-weg : compare a prep.¹: the 14th and 15th cents. forms in o-, on-, were northern; in Middle English and modern dialects reduced to ‘way (York Plays, do way = put away, Scots co’way, c’way = come away), also in certain combinations, as way-going. … In earlier English used as a separable verbal prefix, standing before the verb, especially in subordinate sentences, and complex verb phrases (as in German), e.g. Sone se ich hit awei warp, soon as I threw it away: he wes awæi ifloȝen, he had flown away; still placed emphatically before the subject as ‘away he went,’ ‘away went hat and wig.’

“away, adv.” OED Online, Oxford University Press, September 2019, http://www.oed.com/view/Entry/13905. Accessed 25 September 2019.

So, it is only an adverb, and the OED has fourteen different uses of it (with many a jaunty sub-use), but I was searching for it’s verbal use, as in the dwarves’ song! “We must away…” – clearly it must also do a little time as a verb? No. I discovered this most delicious of explanations:

IV. Elliptical uses, with a verb suppressed: simulating an imperative or (rarely) infinitive.
 14. = Go away.

“away, adv.” OED Online, Oxford University Press, September 2019, http://www.oed.com/view/Entry/13905. Accessed 25 September 2019.

• 1.006 He had been away over The Hill
• 1.010 and floated away over The Hill.
• 1.012 and wanted him to go away.
• 1.023 Then he strode away,
• 1.060 (his appetite was quite taken away),
• 1.068 and put away
• 1.070 and was swept away into dark lands under strange moons,
• 1.073 We must away ere break of day
• 1.077 We must away, ere break of day,
• 1.082 We must away, ere break of day,
• 1.084 until all the dwarves had gone away.
• 1.123 and carry away people,
• 1.124 When I asked how they had got away,
• 1.124 After that we went away,
• 1.128 And Thrain your father went away on the twenty-first of April,
• 1.132 Your father went away to try his luck
• 1.144 We must away, ere break of day,
• 2.035 and all the baggage that he carried was washed away off him.
• 2.048 he wished himself a hundred miles away,
• 2.061 before he remembered not to give his friends away.
• 2.111 going away through the trees.
• 2.117 and carried away the pots of gold,
• 2.125 they had frightened everyone away from the district,
• 3.001 not far away on either side.
• 3.050 Now they rode away amid songs of farewell
• 4.002 Far, far away
• 4.034 Take them away to dark holes full of snakes,
• 5.019 if only I can get away.’
• 5.022 and was driven away, alone,
• 5.067 Guess away!’ said Bilbo.
• 5.083 relieved to think of Gollum going away.
• 5.084 Not far away was his island,
• 5.090 and wailing away
• 5.127 Bilbo crept away from the wall
• 5.128 He must get away,
• 5.145 scrambled away on all fours,
• 6.004 on the other side the ground sloped away
• 6.040 rolled away from their feet;
• 6.040 and they were sliding away,
• 6.045 though far away through the trees
• 6.049 All of a sudden they heard a howl away down hill,
• 6.049 It was answered by another away to the right
• 6.049 then by another not far away to the left.
• 6.063 So the Wargs had no intention of going away
• 6.065 till their own friends chased them away
• 6.069 and they flew away from the mountains,
• 6.075 Fly away little birds!
• 6.075 Fly away if you can!
• 6.076 Go away! little boys!’
• 6.082 or drove them far away;
• 6.092 and flew away with him into the night leaving Bilbo all alone.
• 6.094 and Bilbo far away
• 7.002 and the world was far away,
• 7.012 who lives not far away.
• 7.050 Go on, call away!’
• 7.055 away east beyond Mirkwood,’ put in Gandalf,
• 7.088 and they lived a good way away;
• 7.096 and the voices seemed to grow far away,
• 7.116 At last Gandalf pushed away his plate
• 7.117 only ones going away from here.
• 7.133 away to the right,
• 7.142 some pressing business away south;
• 7.145 As they went away Bilbo could have sworn
• 7.154 Then he galloped away
• 7.154 and go away!’
• 8.010 How far away do you think it is?’ asked Thorin,
• 8.030 thrusting the boat away from the bank,
• 8.062 not far away were scores of twinkling lights,
• 8.070 begun not far away –
• 8.073 all noise at last died right away,
• 8.075 or it would have hurried away quicker.
• 8.081 like a patch of midnight that had never been cleared away.
• 8.094 Bilbo, however, soon slipped away
• 8.094 away from the dwarves,
• 8.104 It had thought of starting the feast while the others were away,
• 8.112 and were dragging him away.
• 8.119 and ‘Attercop’ from among the trees away on the right.
• 8.119 Away behind them now the shouting
• 8.123 their prey moved slowly but steadily away.
• 8.130 and carried him away.
• 8.143 Take him away
• 9.011 even tucked away
• 9.012 I am like a burglar that can’t get away,
• 9.018 the roof had been cut away
• 9.018 were brought from far away,
• 9.022 The guards were tramping away down the passages
• 9.050 and bobbing away down the current.
• 9.053 South away! and South away!
• 9.053 South away! and South away!
• 9.053 South away! and South away!
• 9.053 South away! and South away!
• 9.055 and their voices faded away.
• 9.056 and away.
• 9.061 and tubs away to the north bank,
• 9.063 but he was given away
• 9.068 and went sailing away down,
• 10.001 Suddenly the cliff fell away.
• 10.002 And far away,
• 10.004 if he had known that news of this had reached Gandalf far away
• 10.006 Only from the map did Bilbo know that away up there,
• 10.006 and ran away hurriedly to unknown lands.
• 10.009 and towed away round the high shoulder of rock
• 10.009 and take some of the casks away,
• 10.018 Other folk were far away;
• 10.021 and all the waters of the lake turn yellow right away.
• 11.003 slanting away from the River Running,
• 11.004 died away to a plodding gloom.
• 11.011 But he might be gone away some time,
• 11.021 and go away.
• 11.021 and stare away west through the opening,
• 12.010 till all sign of the door behind had faded away.
• 12.013 stretching away across the unseen floors,
• 12.015 To say that Bilbo’s breath was taken away
• 12.033 Nor could they get away just now,
• 12.034 for bringing away a cup
• 12.057 But don’t let your imagination run away with you!’
• 12.078 Mr. Baggins’ one idea was to get away.
• 12.090 and flew away.
• 12.100 and getting up he kicked away the stone that wedged the door.
• 12.104 and went away south
• 13.018 as he wandered away into the vast hall;
• 13.022 But soon they saw it far away
• 13.033 Smaug was away from home.
• 13.061 We must move away from here,’
• 14.016 hoping to row away
• 15.011 and immediately flew away.
• 15.029 away south
• 16.014 not far away.’
• 16.035 and threw away the wrapping.
• 17.046 rolling away to the South-East;
• 18.020 Then Bilbo turned away,
• 18.041 Then he turned away.
• 18.053 There far away was the Lonely Mountain
• 19.018 he rode away with Gandalf.