MOB is a slur used to describe Americans in a derogatory way. Morbidly Obese Bodies on account of Americans are mostly fat (also could be Morbidly Obese Bitches for women)
The word MOB is a racial slur that has been used historically to degrade and dehumanize americans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to americans. If you are unsure about whether or not it is appropriate to use the word MOB, it is best to err on the side of caution and avoid using it in any situation.