aboriginal :: being the first of its kind in a region: primitive: native
abortive :: unsuccessful: fruitless
abstemious :: temperate: sparing in drink, etc.
abstruse :: obscure: profound: difficult to understand
acetic :: vinegary
acidulous :: slightly sour: sharp: caustic
acquiescent :: accepting passively
acrid :: sharp: bitterly pungent
acrimonious :: stinging: caustic
actuarial :: calculating; pertaining to insurance statistics
adamant :: hard: inflexible
addle :: rotten: muddled; crazy
adept :: expert at
adipose :: fatty
adroit :: skillful
adventitious :: accidental: casual
adverse :: unfavorable: hostile
aesthetic :: artistic: dealing with or capable off appreciation of the beautiful
affected :: artificial: pretended
agape :: openmouthed
aggregate :: sum: total
aghast :: horrified
albeit :: although
alimentary :: supplying nourishment
alluvial :: pertaining to soil deposits left by rivers, etc.
aloof :: apart; reserved
ambiguous :: doubtful in meaning
ambulatory :: able to walk
amenable :: readily managed; willing to be led
amiable :: agreeable; lovable
amicable :: friendly
amorphous :: shapeless
amphibian :: able to live both on land in water
ample :: abundant
amuck :: in a state of rage
analogous :: comparable
ancillary :: serving as an aid or accessory; auxiliary
animated :: lively