[1] : WordNet (r) 2.0

take aback
     v : surprise greatly; knock someone's socks off; "i was floored
         when i heard that i was promoted" syn: shock, stun,
         floor, ball over, blow out of the water
see also:
shock stun floor ball over blow out of the water 

[2] : Moby Thesaurus II by Grady Ward, 1.0

19 moby thesaurus words for "take aback":
   abash, appall, astound, bowl down, bowl over, confound, discomfit,
   disconcert, dismay, electrify, frighten, jar, jolt, put out, shake,
   shock, stagger, startle, stun

Aback >> A Words
Aback, definition of term: Aback
aback_pag1.html

Double Take >> D Words
Double Take, definition of term: Double Take
double+take_pag1.html