soothsayer (n.)

"truth, reality, fact," Old English soð "truth, justice, righteousness, rectitude; reality, a true situation, certainty," noun use of soð (adj.) "true, genuine, real; just, righteous," originally *sonð-, from Proto-Germanic *santhaz (source also of Old Norse sannr, Old Saxon soth, Old High German sand "true," Gothic sunja "truth"). Compare forsooth.

The group is related to Old English synn "sin" and Latin sontis "guilty" (truth is related to guilt via "being the one;" see sin (v.)), from PIE *hes-ont- "being, existence," thus "real, true" (from present participle of root *es- "to be"), also preserved in Latin sunt "they are" and German sind.

Archaic in English, it is the root of modern words for "true" in Swedish (sann) and Danish (sand). It was in common use until mid-17c. then obsolete until revived as an archaism early 19c. by Scott, etc. It was used for Latin pro- in translating compounds into Old English, such as soðtacen "prodigy," soðfylgan "prosequi."

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.

"foretell the future, make predictions," c. 1600, back-formation from soothsayer. Compare Old English soðsecgan "say or speak truly," soðsagu "act of speaking the truth," and verbal phrase secgað soð; also compare soothsaw, Middle English sothesawe, and Old Norse sannsaga. As a noun from 1540s, "a true or wise saying." Related: Soothsaid; soothsaying (1530s).