Douglas Harper's Etymology Dictionary
rantallion
"One whose scrotum is so relaxed as to be longer than his penis, i. e. whose shot pouch is longer than the barrel of his piece." ["Dictionary of the Vulgar Tongue," Grose, 1785]
Wiktionary
rantallion
n. (context slang English) ''(ca. 1780-1850):'' A person whose penis is insufficiently long, in its 'relaxed' mode, to exceed the length of the scrotum