Green’s Dictionary of Slang

whop v.

also wap, whap, wop
[? SE quap, to beat, to throb, ult. Ger. quappen, to flop, quappeln, to quiver]

1. [late 16C+] to hit, to beat, to flog; also fig. use; thus whapper, woppern.

2. [mid-17C] to have sexual intercourse.

3. [mid-19C+] to overcome, to surpass, to defeat.

4. [1910s] (US) to shoot.

In phrases

whop it up (v.) (also whap it up, whop it in)

[1960s+] of a man, to have sexual intercourse; esp. in I could whop it up her/that, I would like to have sex with that woman.