gainsay v.

late Old English agan, from earlier ongean (prep.) "toward; opposite, against, contrary to; in exchange for," as an adverb "in the opposite direction, back, to or toward a former place or position," from on "on" (see on (prep.) and compare a- (1)) + -gegn "against, toward." This is from the Germanic root *gagina (source also of Old Norse gegn "straight, direct;" Danish igen "against;" Old Frisian jen, Old High German gegin, German gegen "against, toward," entgegen "against, in opposition to").

In Old English, eft (see eftsoons) was the main word for "again," but this often was strengthened by ongean, which became the principal word by 13c. Norse influence is responsible for the hard -g-. It was differentiated from against (q.v.) 16c. in southern writers, again becoming an adverb only, and against taking over as preposition and conjunction, but again clung to all senses in northern and Scottish dialect (where against was not adopted). Of action, "in return," early 13c.; of action or fact, "once more," late 14c.

Middle English seien, from Old English secgan "to utter, inform, speak, tell, relate," from Proto-Germanic *sagjanan "to say" (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").

Watkins has this from a PIE *sokwyo-, from a 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"). Some further see this as identical to the PIE root *sekw- (1) "to follow" (with semantic development to "see" and then "speak"). But others doubt it and Boutkan gives it "No certain PIE etymology."

The past tense form said developed from Old English segde. Impersonal use (it is said) was in Old English. The notion in shall we say, etc. (1580s) is "suppose, take for granted." On that analogy, impersonal say is used as an introduction word, or parenthetically, with a clause and meaning "suppose, assume" (c. 1600). Its colloquial use as an expression of surprise, etc. is by 1830.

Not attested before 1930 in use with inanimate objects (clocks, signs, etc.) as subjects. You said it! "you're right" is attested by 1919; you can say that again as a phrase expressing agreement is by 1925, American English colloquial. You don't say(so) as an expression of astonishment (often ironic) is attested by 1779. The Society of American Florists' advertising slogan say it with flowers dates to 1918 from it grew other say it with constructions.