Green’s Dictionary of Slang

(US) bell-bottomed trousers.

1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 231: They look goofy in their ding-dong pants.
1972		B. Rodgers Queens’ Vernacular 22: If his homosexual tendencies have surfaced while he sails for his country, he [...] is prone to unfastening all thirteen buttons on his draperies, ding-dongs (fr naut sl.), ND’s (short for NDBB = Navy Denim Bell Bottoms) [...] and willing to do it.