for 'kiss my ass' from whence came the name of the band the Pogues
Old Irish "póc" comes from the Latin "osculum pacis" (kiss of peace). Latin "pax, pacis" originally meant "a binding together by treaty or agreement", seen as the basis of peace. The word "pax" is derived from the Indo-European root *pak- (to fasten).
Old Irish "tón" comes from Common Celtic *tûknâ(?) (compare Welsh "tin" (rump, rectum)), which derives from Indo-European *teuk-, an extended from of the root *teu- (to swell, be fat). English words derived from this root include "thigh", "thumb" and "tuber".
Everlasting thanks to Dennis King, and the website maintained by James Raum at http://www.lincolnu.edu/~focal/index.html