My assumption is that "Tòn" is just butt, but since that sounds a wee bit to vulgar, they made buttocks out of it (btw, you can still use "butt" and most questions will accept it). Anyhow, since buttocks come in pairs and butts don't, the plural is a bit weird. According to https://en.wiktionary.org/wiki/t%C3%B2n tòn refers to anus, rectum, backside, arse. The plùral of the words tònan. Careful tho, tònan apparently means faggot.  Now, according to learngaelic.net the word ceòs means buttocks , which I assume would be the more specific word for the actual cheeks.
Hopefully somebody native can confirm.