What is the X in X-mas?
word-forming element in legal English (and in imitation of it), representing the Anglo-French -é ending of pps. used as nouns. As these sometimes were coupled with agent nouns in -or, the two suffixes came to be used as a pair to denote the initiator and the recipient of an action.
used to form nouns The object of what is indicated: baby-sittee/ kickee/ muggee