So I was looking up “bum” on etymonline, as one does ...
bum (n.1) Look up bum at Dictionary.com
“buttocks,” late 14c., “probably onomatopœic, to be compared with other words of similar sound and with the general sense of ‘protuberance, swelling.’ “ [OED]
We what now?
In what sense could “bum” be onomatopœic? The mind fairly boggles.