The history of the word is crazily beautiful, it seems to come from Slavonic and Finnic roots, and from the same source as “kin.” “Leader of a kinship band” may be the closest we’ll get to the sense of its first uses.

• 1.075 For ancient king and elvish lord
• 1.094 and knocked their king Golfimbul’s head
• 1.122 and my grandfather was King under the Mountain again,
• 1.122 Kings used to send for our smiths,
• 2.038 They have seldom even heard of the king round here,
• 3.033 as venerable as a king of dwarves,
• 3.035 Foe-hammer that the king of Gondolin once wore.
• 4.004 and valleys where no king ruled.
• 5.055 Slays king, ruins town,
• 7.009 in after days the King of All Birds
• 8.048 A woodland king was there with a crown of leaves,
• 8.071 sat a woodland king with a crown of leaves upon his golden hair,
• 8.132 there lived at this time their greatest king.
• 8.132 In fact the subjects of the king mostly lived
• 8.132 The king’s cave was his palace,
• 8.134 The king looked sternly on Thorin,
• 8.135 at their merrymaking?’ asked the king.
• 8.141 asked the king angrily.
• 8.143 said the king.
• 8.145 There in the king’s dungeon poor Thorin lay;
• 9.003 The king had ordered them to make haste.
• 9.003 to the king’s doors.
• 9.004 before the great gates of the king
• 9.008 O king?’ said Balin,
• 9.009 made the king angrier than ever,
• 9.011 sometimes with the king at their head,
• 9.014 of telling the king
• 9.015 to the king,
• 9.015 with promises to the king of a share
• 9.018 These opened upwards into the king’s cellars.
• 9.018 and especially their king,
• 9.019 listening to the talk of the king’s servants,
• 9.022 Then Bilbo heard the king’s butler
• 9.024 and see if it is fit for the king’s table.
• 9.025 but for the king’s feasts only,
• 9.028 goodness knows where the king will put you next,
• 9.029 Nearly all the king’s folk were merrymaking.
• 9.037 to the king’s palace.
• 9.048 if the king’s full buttertubs
• 9.061 in haste from the king’s great doors.
• 9.069 They had escaped the dungeons of the king
• 10.003 and the river was guarded by the Wood-elves’ king.
• 10.007 in the king’s cellars.
• 10.020 son of Thror King under the Mountain!’
• 10.029 son of Thror King under the Mountain!
• 10.031 These are prisoners of our king
• 10.032 than the return of the King under the Mountain,
• 10.033 not to the raft-men of the king.’
• 10.034 of the King under the Mountain;
• 10.035 The King beneath the mountains,
• 10.035 The King of carven stone,
• 10.036 and they began to think their king might have made a serious mistake.
• 10.040 in the king’s palace.
• 10.040 In any case the king knew now the dwarves’ errand,
• 10.043 of the old kings.
• 10.043 was really the grandson of the King under the Mountain,
• 12.072 The King under the Mountain is dead
• 12.096 of the great King Bladorthin
• 12.103 who is the real King under the Mountain!’
• 14.005 Perhaps the King under the Mountain
• 14.006 Which king?’
• 14.006 the only king under the Mountain we have ever known.’
• 14.008 The King beneath the Mountain!’
• 14.022 of the true king under the Mountain,
• 14.027 we would make him a king.
• 14.030 King Bard!
• 14.030 King Bard!’
• 14.031 not king of Esgaroth,’
• 14.031 Let ‘King Bard’ go back to his own kingdom –
• 14.032 We will have King Bard!’
• 14.034 of a new king,
• 14.039 to ask the aid of the King of the Elves of the Wood,
• 14.041 said the king.
• 14.042 But the king,
• 15.014 but we remember still the king that was of old.
• 15.032 King under the Mountain,
• 15.036 The King has come unto his hall!
• 15.040 The king of friend and kin has need.
• 15.041 Here at the Gates the king awaits,
• 15.042 The king is come unto his hall
• 15.045 King under the Mountain?’
• 15.056 calling himself the King under the Mountain,
• 16.022 I know your king well by sight,
• 16.030 asked the king.
• 16.042 and as he went both the king
• 17.024 as King under the Mountain,’ said Gandalf.
• 17.054 and the elf-lords were at bay about their king
• 17.055 Out leapt the King under the Mountain,
• 18.018 Farewell, King under the Mountain!’
• 18.032 and he became King under the Mountain,
• 18.034 greater than that of many mortal kings.
• 18.043 even though the king bade them stay a while
• 18.045 said the king.
• 18.047 said the king.
• 18.049 said the king gravely.

“king, n.” OED Online, Oxford University Press, September 2019, http://www.oed.com/view/Entry/103510. Accessed 30 October 2019.