whilstwhilst

The story

From Late Middle English whilst, whilest, qwhilste (Northern England), quilest (Northwest Midlands) [and other forms], from whiles (‘during the time that, while; only so long as; provided that; because, since; until’) + -t (excrescent suffix, perhaps due to a combination of -(e)s and the following word the, or influenced by the superlative suffix -est). Whiles is derived from whiles (“period of time, a while”, noun) (probably from the second element of adverbs and conjunctions like otherwhiles and somewhiles), from while (“period of time, a while”, noun) + -s (suffix forming adverbs of manner, space, and time); and while is from Old English hwīl (‘period of time, a while’), ultimately from Proto-Germanic *hwīlō (‘period of time, a while; period of rest, break, pause’), from Proto-Indo-European *kʷyeh₁- (‘to rest; peace, rest’). The English word can be analysed as whiles + -t (excrescent suffix appended to words suffixed with -s).

Full etymology on Wiktionary