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1.
The jocular act of hitting someone in the face, usually gently, with a cold, plastic milk bottle. The bottle is generally at least 6 pints – large enough to cover one side of a person's face.

This action often just cools the victim's face in a pleasant way, rather than actually hurting them.
It's a milk slap, L.B.!
by Kokapetl January 19, 2009