I don't really know, but I know that the first memorable shot that sticks in my mind was Under Siege. I'm positive I'd seen them prior to that, but that's one that I remember most.
I can't remember for sure what the first was. It could have been the scene in Airplane, or a cable show called Bizzarre with John Biner. There was also some other strange showtime cable series that had them. Animal House maybe was a possibility too. Also Bruce Lee movies usually had one scene in them. I saw the movie Excalibur which also had nudity. The first R rated film I saw was Billy Jack and The born Losers(my brothers took me when I was a kid), but I can't remember if there were boobs in it or not. Probably not.
From this thread, we can deduct who's in their respective age bracket. Younger dudes would have seen b00bs in recent movies. UNDER SIEGE?!?!!? That was in the '90s!! Does anyone recall if EXTRO had any b00b scenes? If so, I stand corrected on seeing them on PORKY'S REVENGE first.
HAIL THIS MOVIE!! Funny thing is, my GRANDMOTHER bought it for me when I was like, 11. I'm pretty sure she had no idea it was R-rated. Was it EVER R-rated! Oh man. My pants feel funny even now thinking about it. I'm in my late 20s, so there had already been a few sequels by the time I'd seen this.
If it does, then that would be my first also. If not, how about The Deep? Jacqueline Bisset's wet tshirt and if neither of those count...then probably some b-grade scream fest on cable in the early 80s.