(exclamatory phrase) a playful attempt at indimidation which follows an insult
popularized by DT, OH ZIP is derivative of OH SNAP where an insult is followed up by this indication of surprise. While derivative of "oh snap" OH ZIP has more of an emphasis on intimidation and plays off the implication that the speaker is unzipping his pants to reveal an larger-than-normal appendage.
see also: OH SNAP
Lyneah: DT, you're looking pretty weak in your old age.
DT: OH ZIP! ::unzips pants::
Lyneah (amused): haha, touche!