To consent to sexual relations with another; primarily refers to women; see also hit (the) skins

For a woman to allow her sex partner to have intercourse with her.

the event of sharing skin with hopefully the opposite sex.

When two friends meet and touch their hands together in a special handshake/greeting. (one hand may be slapped down onto the others, a 'high five' action, or two fists colliding in a friendly punching action) Usually said as a command.