1. Present participle of dip.
2. The act of inserting one's (usually unprotected) penis briefly into the vagina before withdrawing, as a method of foreplay or before she realises what's happening.
3. The hetro- and homosexual anal equivalent of this.
Dipping is a further development of freak dancing. The girl unzips the boy to enable a dip, usually one brief shallow penetration in time to the music.
It is not considered sex, which is far more prolonged. It is more like giving each other a brief "kiss" with the genitals. If a boy tries to prolong a dip, he is avoided for at least the rest of the evening by all the girls at the dance.