flying sixty-six, n. — Green’s Dictionary of Slang

flying sixty-six n.

[rhy. sl. = French tricks under French adj. but note flying sixty-nine under flying adj.]

oral sex.

1980

Maledicta IV:2 (Winter) 197: If, though, she should be afraid of getting clucky [...] she may be satisfied with French kissing, French tricks or a flying 66 (rhyming slang on French tricks).