Green’s Dictionary of Slang

1. an orgasm. an ejaculation.

1965		Guild Dict. Homosexual Term 33: [Simes:DLSS].
c.1972		‘D. Baxter’ His Big Brother 130: Sloane’s [penis] was just as hard as ever. The blast-off it had performed up David’s ass hadn’t softened it in the least [Simes:DLSS].
1984		Leyland & Juddell (eds) Hot Acts 219: I quickened my motions as his cock reached blast-off [Simes:DLSS].
1994		Men’s Rooms (NY) Feb. 124/2: [S]nce I was still a couple of minutes to blast-off myself, I slowed my pumping [Simes:DLSS].

2. a thrill, a very good time.

1969		Jonathan Winters Show [TV show] It’s going to be a real blast-off [HDAS].
1979		E. Torres After Hours 169: After the speeches, Macho, Puente, Celia, all of them got together and worked a jam session. Blast-off! Thought the joint was gonna fall down.

3. attrib. use of sense 2.

1991

P. Kael Movie Love 274: Nichols directs Shirley MacLaine to overdo the mother’s self-preoccupation; it’s a blast-off performance.