The OED calls this a rare word, not archaic, when used in its figurative sense, as here:

• 12.068 but poor Bilbo was really very taken aback.

“Aback” is “backward from the action” and to take aback, therefore, is to discomfit.  The “a-” prefix here is a descendant of the unaccented prefix “on-“, a particle which forms verbs, adverbs, and prepositions with the sense of “on”.

“a-, prefix3.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“aback, adv.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“take, v.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.