O.E. seldum, alteration of seldan "rare," on analogy of adverbial dative plurals in -um (e.g. whilom "at one time"), from P.Gmc. *selda- "strange, rare" (cf. O.N. sjaldan, O.Fris. selden, Du. zelden, Ger. selten), perhaps ult. from the base of self (q.v.).