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"utterance, recitation, action of the verb 'say,' " c.1300, verbal noun from say (v.); meaning "something that has been said" (usually by someone thought important) is from c.1300; sense of "a proverb" is first attested mid-15c.
Ça va sans dire, a familiar French locution, whose English equivalent might be "that is a matter of course," or "that may be taken for granted." But recently it has become the tendency to translate it literally, "that goes without saying," and these words, though originally uncouth and almost unmeaning to the unpractised ear, are gradually acquiring the exact meaning of the French. [Walsh, 1892]
Old English secgan "to utter, inform, speak, tell, relate," from Proto-Germanic *sagjanan (cf. Old Saxon seggian, Old Norse segja, Danish sige, Old Frisian sedsa, Middle Dutch segghen, Dutch zeggen, Old High German sagen, German sagen "to say"), from PIE *sokwyo-, from root *sekw- (3) "to say, utter" (cf. Hittite shakiya- "to declare," Lithuanian sakyti "to say," Old Church Slavonic sociti "to vindicate, show," Old Irish insce "speech," Old Latin inseque "to tell say").
Past tense said developed from Old English segde. Not attested in use with inanimate objects (clocks, signs, etc.) as subjects before 1930. You said it "you're right" first recorded 1919; you can say that again as a phrase expressing agreement is recorded from 1942, American English. You don't say (so) as an expression of astonishment (often ironic) is first recorded 1779, American English.
"what someone says," 1570s, from say (v.). Meaning "right or authority to influence a decision" is from 1610s. Extended form say-so is first recorded 1630s. Cf. Old English secge "speech."