出典:Wiktionary
From Middle English overflowen (過去分詞 of overflowen besides まれに overflowed), from 古期英語 oferflōwen (過去分詞 of oferflōwan)
overflown
The use of the past participle overflown for overflow sometimes occurs in American English, where it is often proscribed and seen as resulting from a confusion of overflow and overfly. However, it was once more prevalent throughout the English-speaking world, and is ultimately the original form.
「overflown」は動詞「overfly」の過去分詞です