OMG, it's now official
And so are LOL (laughing out loud), IMHO (in my humble opinion), and BFF (best friends forever).
One would imagine that the internet has a lot to answer for. However, the OED relates that the first confirmed use of OMG was in 1917.
Muffin top? Apparently, it is the unpleasant roll of flesh that forms over the waistband or hipband of an overly tight pair of pants.