Green’s Dictionary of Slang

1. (US surfing) a huge wave.

1961		F. Kohner Gidget Goes Hawaiian 50: Then the first real boomers came on and we turned our boards.
1979		D. Maitland Breaking Out 235: You’ve gotta wait around a whole day for the bloody tide to come back up. You’ll see! I’ll be shooting a bloody boomer tomorrow!

2. (US) a thunderstorm, a thunder-cloud.

1979

S. King Long Walk in Bachman Books (1995) 246: ‘It’s going to be a boomer!’ Parker yelled [...] They could see the curtain of rain beating across the woods toward them below the purple thunderheads.

3. in fig. use, something impressive.

1972

D. Jenkins Semi-Tough 9: T. J. got naked and laid down on the bed and spread out his big old pink legs with his ass facing the bathroom door. ‘I was gonna give her my Class A boomer.’.