CAP: The American Sign Language (ASL) sign for "cap"

Mime putting on a baseball cap.

The sign for "CAP" can also be used to express the concept of "putting a hat on"

CAP:
:

If you are referring to a baseball cap or ball cap you would sign:

If you want to convey the concept of "cap" as in "put a lid on" you would sign:

Notes: 

See: HAT

See: LID

See: CLOTHES