Green’s Dictionary of Slang

ding-dongs n.

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

(US) the testicles.

[US]E. Brown Fire 98: A man gets his dong-dongs kicked off going up to the cockpit [HDAS].
[US]B. Rodgers Queens’ Vernacular.
[[US]L. Frank Playing Hardball 118: The protective metal device for the groin; ‘ding-dong!’].
[US]H. Max Gay (S)language.