Green’s Dictionary of Slang

agates n.

also aggots
[SE agate, a semi-precious stone]

(US) the testicles .

[US] ‘Miscellany’ in AS XVI:3 236: Pig testicles are agates, sheep testicles mountain oysters.
[NZ]R. Helmer Stag Party 40: Pickle me bloody agates, eh. Never thought I’d see the day.
[Aus](con. 1940s–60s) Hogbotel & ffuckes ‘Shearer’s Lament’ in Snatches and Lays 73: I took off his prize ram’s aggots.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 17: Aggots Testicles.
J. & E. Coen Big Lebowski [film script] He says ‘My wife’s a pain in the ass. She’s always busting my friggin’ agates.’.
[US]Darcy ‘Into Temptation’ [Internet] Her hand shot out and grabbed his testicles, squeezing them hard [...] He leaned closer to Elizabeth now and softly kissed her on the cheek. ‘Suppose I say you’re right. Will you let go of my agates, then?’.