Green’s Dictionary of Slang

thrups n.1

also thrupennies
[play on thrups n.2 ; ult. rhy. sl. on threepenny bits n. (2) = tit n.2 (1)]

the breasts.

[UK]K. Sampson Powder 440: I haven’t told youse about her thrupennies, yet . . . fucking pure silicone!
[Ire]P. Howard Curious Incident of the Dog in the Nightdress 280: Lovely Libby with the humungous thrups.