Green’s Dictionary of Slang

dong n.1

[abbr. ding-dong n.3 , which predates general regional sl. use]
(orig. US)

1. [late 19C+] (also dang, dange) the penis.

2. [20C+] a general derog. term: a fool.

3. [2000s] a dildo.

In phrases

dong and gongs (n.)

[1980s] (US gay) the male genitalia.

doubledong(er) (n.)

[1990s+] (US) a dildo with two penis-shaped ends which can be used simultaneously by two people, usu. women.

flog one’s dong (v.)

[20C+] to masturbate.