adjective

1. having a sharp edge or point; that can cut well a keen knife, a keen edge
2. sharp or cutting in force; piercing a keen appetite, a keen wind
3. sharp and quick in seeing, hearing, thinking, etc.; acute keen eyes, a keen intelligence
4. sharp-witted; mentally acute; shrewd
5. eager; enthusiastic; much interested: often with about, on, etc.
6. strongly felt or perceived; intense; strong keen desire, a keen scent
7. ☆ Slang good, fine, etc.: a generalized term of approval

Etymology: ME kene < OE cene, wise, learned, akin to Ger kühn, bold < IE base *ĝen-, to know: the principal senses spring from the basic notion “capable”