The use of bum to mean buttocks is common in the UK and Australia.
I’ve only heard this usage from one American source: Family Guy.
“Besides, you look like you got a clean bum.”
S11E06 Lois comes out of her shell
“Wow! That is awesome, Lois! Your bum looks like an NBA guy’s arm.”
S13E08 Our idiot Brian
“I’ll get the scissors and cut that poop off your bum.”
And several other examples.
Is this just a Seth MacFarlane thing? Does this usage have any real currency in the USA?