What’s the origin of the phrase ‘Well hung’?
The idiom might be thought to be fairly modern but, in fact, its first use was in Britain in the 17th century. Before being applied to humans the expression was used to refer to dogs that had large ears.
Meaning
Euphemism; describing a man with a large penis.

The idiom might be thought to be fairly modern but, in fact, its first use was in Britain in the 17th century. Before being applied to humans the expression was used to refer to dogs that had large ears.
UsageWorldwide.
From the Phrase Finder idioms collection, written by Gary Martin.
Historical trend
Source: Google Books Ngrams (1820–2020).