BMSHOMP&J is an abbreviation much like "lol", "wtf", or "omg". It stands for "black man style hand over mouth pointing & jumping" because sometimes when african american males get excited about something, they cover their mouth with their hand, point at whatever they're excited about and jump up and down, usually yelling "ohhhhhh!"
person 1: dude the song is called "can't see me" by 2pac featuring george clinton, who my friends dad hung out with once and most likely smoked weed with....no biggie
person 2: *BMSHOMP&J* ohhhhhhhh!!!!!!