Origin and history of whilom

"span of time," especially "short space of time during which something is to happen or be done or certain conditions prevail;" Old English hwile, accusative of hwil "a space of time," from Proto-Germanic *hwilo, which is reconstructed to be from PIE *kwi-lo-, suffixed form of root *kweie- "to rest, be quiet." The notion of "period of rest" became in Germanic "period of time."

Now largely superseded by time (n.) but preserved in formulaic constructions (such as all the while). The sense of "time spent in doing something, expenditure of time" is in worthwhile and phrases such as worth (one's) while.

As a conjunction, "at the same time that; as long as" (late Old English), it represents Old English þa hwile þe, literally "the while that."

The form whiles (with adverbial genitive) is recorded from early 13c.; whilst is from late 14c., with unetymological ending as in amongst, amidst. Service while-you-wait is attested from 1911. Wycliffe (1384) translates Latin vicissitudo with whileness.

Germanic cognates include Old Saxon hwil, Old Frisian hwile, Old High German hwila, German Weile, Gothic hveila "space of time, while"), originally "rest" (compare Old Norse hvila "bed," hvild "rest."

"rarely, not often, infrequently," late Old English and early Middle English seldum, an alteration of seldan "infrequently, rarely," from Proto-Germanic *selda- "strange, rare" (source also of Old Norse sjaldan, Old Frisian selden, Dutch zelden, Old High German seltan, German selten), a word of uncertain etymology. Perhaps ultimately from the base of self (q.v.).

The form shifted apparently on analogy of adverbial dative plurals in -um (as in whilom "at one time," from Old English hwilum, from the source of while). The same development also created litlum from little, miclum from mickle. Also compare random, ransom. The forms in -n decreased from 14c. and faded out in 16c.

Old English seldan had comparative seldor, superlative seldost; in early Middle English, as seldan changed form and lost its connection with these, selde was back-formed as a positive. Shakespeare uses seld-shown "rarely exhibited." 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). 

 German seltsam "strange, odd," Dutch zeldzaam are cognates of seldom, but with the second element conformed to their cognates of -some. Related: Seldomness.

Seldom-times "rarely, hardly ever" is from mid-15c. (earlier was seld-when, Old English seldhwanne "rarely," which lasted until 16c.). Seldom-seen "rarely encountered" is from mid-15c.; older was seld-seen (Middle English seld-sen, from Old English seldsiene), which lasted long enough to appear in Marlowe (seildsene, 1590s).

*kweiə-, also *kwyeə-, Proto-Indo-European root meaning "to rest, be quiet."

It might form 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 might also be the source of: 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."