late Old English seldum, alteration of seldan "seldom, rarely," from Proto-Germanic *selda- "strange, rare" (cf. Old Norse sjaldan, Old Frisian selden, Dutch zelden, Old High German seltan, German selten), perhaps ultimately from the base of self (q.v.).
Form shifted on analogy of adverbial dative plurals in -um (e.g. whilom "at one time," from while). The same development also created litlum from little, miclum from mickle. German