Origin and history of whilst

a variant of amid (q.v.) with adverbial genitive -s and unetymological -t. Amidde became amyddes (13c.) and acquired the -t from mid-15c., probably by association with superlatives in -st; the pattern also yielded amongst, against, betwixt, whilst, also archaic alongst (13c.-17c.).

There is a tendency to use amidst more distributively than amid, e.g. of things scattered about, or a thing moving, in the midst of others. [OED]

"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Now largely superseded by time (n.) but preserved in formulaic constructions (such as all the while).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."