mfsoj Share definition Flag An acronym of "Mother Fucking Son of Jesus", used when there is too little time or it's too big an effort to use the full phrase. A: Look at that cute polar bear. B: MFSOJ! it's going right for us ! mfsoj by BaQ December 9, 2008 0 0 Get merch Get the mfsoj mug. Buy Now — $32.95 See more merch