To
Iceberg Slim and his
contemporaries, this was
jivespeak for a small
glycerin container for holding
drugs. You might put some
girl in a
cap, then store it under the floor in your
crib.
For Slim's more modern colleagues, this is a verb meaning 'to kill'. If you see someone on your turf that's not wearing your colors, it may be a good idea to cap them.