It means "sharp", honed, and it refers to a blade, a person's intelligence or to a feeling. The word generally is not used to describe pointed implements. ----- It also means eager, enthusiastic, impatient, determined, ambitious,avid, ardent, passionate, fervent, fervid, conscientious, committed, dedicated, zealous, acute, incisive, astute, and so on.
The meaning of keen observer is enthusiastic eager watcher or onlooker
To be keen about something
keen
"Keen" is and adjective, used to describe something as sharp or acute. "The sword had a keen edge." "He was a man of keen intellect." It can also be used to express enthusiasm. "He was keen to learn another language."
alert. avid, agog
It means fervent, zealous, eager, passionate, fanatical or keen.
Sharp; having a fine edge or point; as, a keen razor, or a razor with a keen edge., Acute of mind; sharp; penetrating; having or expressing mental acuteness; as, a man of keen understanding; a keen look; keen features., Bitter; piercing; acrimonious; cutting; stinging; severe; as, keen satire or sarcasm., Piercing; penetrating; cutting; sharp; -- applied to cold, wind, etc, ; as, a keen wind; the cold is very keen., Eager; vehement; fierce; as, a keen appetite., To sharpen; to make cold., A prolonged wail for a deceased person. Cf. Coranach., To wail as a keener does.
keen kewl: slang for cool.
As keen as mustard or as keen as a kitten
1. Having a fine, sharp cutting edge or point.2. Having or marked by intellectual quickness and acuity.3. Acutely sensitive like example: a keen ear4. Sharp, vivid or strong5. Intense/piercing like example: a keen wind.6. Pungent/acrid like example: A keen smell of skunk was left behind.
The expression is to be "keen on" something or someone, if you are an enthusiast or a lover.
A dog has a keen sense of smell, a bat has a keen sense of hearing, a cheetah has a keen sense of sight