It's not obscure medical jargon, but I doubt you'd hear it in an informal context. I wouldn't think twice if I heard "buttocks" in a doctor's office, on the news, or in a courtroom.
Between two people in a casual conversation though, it would sound strange. 'Butt', is common, 'ass' is common, and neither one is censored on broadcast prime-time TV, so they can't be all that offensive.
The very reason they chose to use the word "buttocks" in Forrest Gump was to make the character sound awkward.
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