an- (1) Look up an- at Dictionary.com
privative prefix, from Greek an-, "not, without," from PIE root *ne "not" (source also of Greek ne- "not," Sanskrit an-, Latin in-, Gothic and Old English un-; see un- (1)). The Greek prefix is a fuller form of the one represented in English by a- (3).
an- (2) Look up an- at Dictionary.com
a later form of Latin ad "to" before -n- (see ad-), as in annex, announce, annihilate.