Rob Gronkowski -- SORRY FOR PARTYING

Rob Gronkowski is SORRY -- sorry for putting his busted forearm in danger last week in Vegas while partying like a dumbass ... just ask Gronk's tank top.

A Patriots fan posted the photo on Twitter this weekend -- showing Gronk wearing a tank that reads, "Sorry for partying." According to the fan, the photo was snapped Sunday at Universal Studios in Orlando.

The shirt's obviously a reference to the video TMZ posted last week, in which Gronk body slams his friend on stage at XS nightclub in Vegas ... and lands hard on his broken left forearm, which was wrapped in a support sleeve.

So, here's your apology, New England. Satisfied?