The headword is a crasis of kai\ e)/xezen, i.e. 'and' plus the third person singular imperfect of the verb xe/zw. The form was likelier to need glossing because of the crasis than for unfamiliarity of the verb itself, since xe/zw has survived into Modern Greek.
[1] Aristophanes, Acharnians 82 (web address 1). Adler indicates that this entry was copied from kappa 184 (ka)/kxezen). Apparently the text of Aristophanes varied between using xe/zw and the compound e)kxe/zw "I shit out" (LSJ s.v.; web address 2).
[2] The euphemism used in the gloss (a)popate/w lit. "to step away", in LSJ. s.v. "retire to ease oneself"), indicates that xe/zw was felt to be vulgar by the lexicographer. (The scholiast of Aristophanes uses a)pedh/mei, "travelled away" for this word.)