ding-dongs, n. — Green’s Dictionary of Slang

ding-dongs n.

[ding-dong n.² (2), i.e. they supposedly knock together, ‘like bells’]

(US) the testicles.

1972		B. Rodgers Queens’ Vernacular.
[1983		L. Frank Playing Hardball 118: The protective metal device for the groin; ‘ding-dong!’].
1988		H. Max Gay (S)language.