MUSH stands for Mulit-User-Shared-Hallucination. It uses a very strange, psuedo-programming language, and is used by nerds to RP on regularly, often early into the morning.

A kiss ( a slang from 1940's )

Mush a feeling so unrealistic to other people, that only you and your partner or friend understand the true meaning of MUSH.
Or
A over powering feeling of affection and emotion for one person that the best way to describe your emotion is MUSH.

to screwup, err or otherwise generally break something, generally due to ignorance. To make something FUBAR

Face. Your Mush is your face or {grill}

A strange northern Irish drink made by mixing fermented bull or horse semen with potatoe wine