From Middle English seyen, seien, seggen, from Old English seċġan ("to say, speak"), from Proto-West Germanic *saggjan, from Proto-Germanic *sagjaną ("to say"), from Proto-Indo-European *sokʷ-h₁-yé-, a suffixed o-grade form of *sekʷ-.
Cognate with West Frisian sizze ("to say"), Dutch zeggen ("to say"), German sagen ("to say"), Danish sige ("to say"), Norwegian Bokmål si ("to say"), Norwegian Nynorsk seia ("to say"), Swedish säga ("to say"), Yiddish זאָגן ("to say; to tell").
The adverb and interjection are from the verb.
From Middle French saie, from Latin saga, plural of sagum.