Prolong

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late Middle English: from Old French prolonguer, from late Latin prolongare, from pro- ‘forward, onward’ + longus ‘long’.

wiktionary

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Either a back-formation from prolongation, or from Old French prolonguer or porloignier, from Latin prōlongō, from prō + longō. Doublet of purloin.

etymonline

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prolong (v.)

early 15c., prolongen, "lengthen in time, extend the duration of; delay, postpone," back-formation from prolongation or else from Old French prolonguer, porloignier (13c.) and directly from Late Latin prolongare "to prolong, extend," from Latin pro "forth" (see pro-) + longus "long" (adj.); see long (adj.). The same elements also form purloin. Related: Prolonged; prolonging; prolongable.