Dull.
vigilant
The word "lean" fits your criteria as it means cool and rhymes with green.
The word "keen" rhymes with mean and can also mean to scream loudly or shout enthusiastically.
Sharp ANSWER 2 Keen means intelligent. It can also be a verb and mean to wail or sob loudly. For example: "The woman saw that her child was dead and began to keen." Keen is also a slang word from the 50's and 60's and it means great, wonderful, good, cool. A teenager might remark, " That is a keen guitar you have."
The abstract noun form for the adjective keen is keenness.
Mustard has a sharp or keen taste. "Keen as mustard" means being sharp-minded.
The root for the word acute would be 'acu-.' This root is Latin, and it means, 'keen, unpleasantly bitter, or sharp.'
There is only one (1) syllable in the word keen.
A 6-letter word beginning with 'a' that means smart is astute.
blunt, pointless
Roy was very keen for apples today.
A knife is a keen tool that is dangerous for kids to play with.