Etymology
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say (v.)
Old English secgan "to utter, inform, speak, tell, relate," from Proto-Germanic *sagjanan (source also of 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" (source also of 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.
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say (n.)
"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. Compare Old English secge "speech."
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says 
third person singular of say (v.), c. 1300, eventually replacing saith.
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said (adj.)
"named or mentioned before," c. 1300, past-participle adjective from say (v.).
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naysayer (n.)

"one who refuses or denies," 1721, from verb naysay (implied from 1530s in naysaying); from nay + say (v.). Nay-say "refusal" is from 1630s.

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soothsayer (n.)
mid-14c., zoþ ziggere (Kentish), "one who speaks truth,;" late 14c., sothseggere, "fortune-teller;" see sooth + say. Old English had soðsagu "act of speaking the truth."
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unsaid (adj.)
Old English unsæd, from un- (1) "not" + past participle of say (v.). Similar formation in Middle Dutch ongeseit, German ungesagt, Old Norse usagðr.
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saying (n.)

"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]
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gainsay (v.)
"contradict, deny, dispute," c. 1300, literally "say against," from gain- (Old English gegn- "against;" see again) + say (v.). In Middle English it translates Latin contradicere. "Solitary survival of a once common prefix" [Weekley]. It also figured in such now-obsolete compounds as gain-taking "taking back again," gainclap "a counterstroke," gainbuy "redeem," Gaincoming "Second Advent," and gainstand "to oppose." Related: Gainsaid; gainsaying.
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hearsay (n.)
"information communicated by another, gossip," mid-15c., from phrase to hear say (Middle English heren seien, Old English herdon secgan). The notion is "hear (some people) say;" from hear (v.) + say (v.). As an adjective from 1570s. Hearsay evidence (1670s) is that which the witness gives not from his own perception but what was told to him. Compare similar formation in Dutch hooren zeggen, German hörensagen.
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