Stands for Pre-Menstrual Tension.
The female gender get it around the time of their period. If you know whats good for you stay away from a female on PMT!! A girl on full blown PMT is not a pretty sight. Approach with care, the slightest action can trigger of a hissy fit and do not do anything to upset her or make her angry.
Its all common sence really =)
Girl - "Omfg!! Dude just fuck off!!! I'm sick fed up of you you're really pissing me off!!! I'm not taking this!"