Green’s Dictionary of Slang

pounder n.

1. in pl., the testicles [? their knocking together].

[UK]T. Shadwell Scowrers Act II: I’ll lay her on board with my two Pounders and my Patereras [i.e. a small gun that fired stones].
[UK]Motteux (trans.) Gargantua and Pantagruel (1927) II Bk IV 410: We would kiss his bare arse without boggling at it, and eke his two pounders; for he has a pair of them, the holy father, that he has.
[US]Maledicta IV:2 (Winter) 196: Terms such as nags and marbles, or pounders, or ladies’ jewels, tarriwags or knackers all refer to the same thing.

2. (US black) a police officer [abbr. beat pounder under beat n.1 ].

[US]Pittsburgh Courier (PA) 8 Feb. 7/1: The pounders up here are doing the indian act because a gate of color had a heated beef with a pale pan and the sepia [...] nixed the gray out.
[US]Mezzrow & Wolfe Really the Blues 32: We just began to eat when in breezed these two pounders on the bloodhound tip.
[US]Murtagh & Harris Who Live In Shadow (1960) 53: Well, at my trial them pounders snapped their caps and skullbusted the judges.
[US]J. Ridley Conversation with the Mann 36: Other times the pounders would come ’round [...] take a wrench back to the hydrant and shut it down.