1. Member of the Opposite Sex
2. Mutual Oral Sex (see 69)
3. Make out session
4. "Mom over shoulder", a warning in chatrooms or IMs
5. Ministry of Sound, a nightclub in London UK
6. A short form for mos def, which is a short form for "most definitely".
7. Military Occupational Specialty (US Army)
M.O.S. can also be an abbreviation of a shitload of other terms that are mos def not used in slang. Including Metal-Oxide Semiconductor, Made of Steel, Marine Observation Satellite, Medical Outcomes Study, Member of Service, Mode Of Shipment etc.
A: "So tell me what happened after the party at MoS last night!"
B: "Ok, I was just about to leave when I met this really handsome MOS.
He told me his MOS is Infantryman in the US Army. Guys in uniform just turn me on!"
A: "Sweet, and then? Did you make out with him?"
B: "Yes! We had a good mos in the club and then he asked me if I wanted to come to his place, I said 'mos!'"
A: "Really, so did you...?"
B: "Yeah, when we got there, we started to have some real good MOS..."
A: "Wait, MOS!"
B: "Ok, ttyl ;)"