It's an abbreiation for the phrase "Ultimate Mind Fuck". It can have 2 possible meanings:
1. when 2 people unite mentally as well as physically during sex. This is implicated in the 1993 song "UMF" by Duran Duran. The terms "UMF" and "Ultimate Mind Fuck" are not in the lyrics.
2. brainwash, mass deceit, censorship, when several people (as in a cult or an entire nation) are mentally
manipulated and hoodwinked and led astray by
propaganda, corruption and outright lies and
absolute bullshit.
1. Renee and Angela made love last night and
experienced union in body and mind. They shared the UMF.
2. In the 90s decade there was a lot of New Age "solutions", shilling evangelists, superstitions, comet cult suicides, Y2K hysteria and other end-of-the-millenium bullshit. Then came the 00s with all its
corruption,
stupidity and shit and the Iraq war circus. That's one major UMF after another!