First attested in the Worin seokbo (月印釋譜 / 월인석보), 1459, as Middle Koreanᄡᆞᆯ (Yale: psol). The Middle Korean p- is still preserved in many compounds where 쌀 is the second element, such as 멥쌀 (mepssal) and 찹쌀 (chapssal).
Vovin (2015) argues that it could be a Japonic loan, connecting it with Japanese 早稲(wase, “early-ripening rice”), with two assumptions:
That the initial p- came from a phonological inability to render initial Japonic w-.
That the final consonant was originally present in early Japonic but had been eliminated in the insular languages.