The term is casual slang and can be impolite or offensive in formal contexts. He casually boasted that he was well hung, which was inappropriate at the formal dinner.
Pro Tip 2/3
Gender Specific
This adjective usually refers to males and is rarely used for females. During the bachelorette party, they joked about looking for a well hung partner.
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Conversational Use
Most likely to be found in informal conversations, jokes, or adult discussions. In the locker room, there were whispers about the new athlete being well hung.