"at time past" (archaic), c. 1200, from Old English hwilum "at times," dative case of while (q.v.). As a conjunction from 1610s. Similar formation in German weiland "formerly."
Entries linking to whilom
Now largely superseded by time except in formulaic constructions (such as all the while). Middle English sense of "short space of time spent in doing something" now only preserved in worthwhile and phrases such as worth (one's) while. As a conjunction, "during or in the time that; as long as" (late Old English), it represents Old English þa hwile þe, literally "the while that." Form whiles is recorded from early 13c.; whilst is from late 14c., with unetymological -st as in amongst, amidst. Service while-you-wait is attested from 1911.
It forms all or part of: acquiesce; acquit; awhile; coy; quiesce; quiescent; quiet; quietism; quietude; quietus; quit; quitclaim; quite; quit-rent; quittance; requiescat; requiem; requite; while; whilom.
It is the hypothetical source of/evidence for its existence is provided by: Avestan shaitish "joy," shaiti- "well-being," shyata- "happy;" Old Persian šiyatish "joy;" Latin quies "rest, repose, quiet;" Old Church Slavonic po-koji "rest;" Old Norse hvild "rest."
late Old English seldum, alteration of seldan "seldom, rarely," from Proto-Germanic *selda- "strange, rare" (source also of Old Norse sjaldan, Old Frisian selden, Dutch zelden, Old High German seltan, German selten), perhaps ultimately from the base of self (q.v.).
The form shifted on analogy of adverbial dative plurals in -um (such as whilom "at one time," from while). The same development also created litlum from little, miclum from mickle. Also compare random, ransom. German seltsam "strange, odd," Dutch zeldzaam are related, but with the second element conformed to their versions of -some.
Seldom-times is from mid-15c. (Old English had seldhwanne "seldwhen"). Seldom-seen is from mid-15c. (Old English had seldsiene, "seld-seen"). Some compounds using the old form survived through Middle English, such as selcouth"rarely or little-known, unusual, strange, wonderful," from Old English selcuð, seld-cuð, from seldan + cuð (see couth). Old English seldan had comparative seldor, superlative seldost; in early Middle English, as seldan changed form and lost its connection with these, selde was formed as a positive. Shakespeare uses seld-shown.