O.E. secgan "to utter, say," from P.Gmc. *sagjanan (cf. O.S. seggian, O.N. segja, O.Fris. sedsa, M.Du. segghen, Du. zeggen, O.H.G. sagen, Ger. sagen "to say"), from PIE *sokei-, probably from base *seq- "point out" (cf. Hitt. shakiya- "to declare," Lith. sakyti "to say," O.C.S. sociti "to vindicate, show," O.Ir. insce "speech," O.Latin inseque "to tell say"). Past tense said developed from O.E. 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, Amer.Eng. You don't say (so) as an expression of astonishment (often ironic) is first recorded 1779, Amer.Eng.
"what someone says," 1570s, from say (v.). Extended form say-so is first recorded 1630s.
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
say in Technology
A human may "say" things to a computer by typing them on a terminal. "To list a directory verbosely, say "ls -l"." Tends to imply a newline-terminated command (a "sentence"). A computer may "say" things to you, even if it doesn't have a speech synthesiser, by displaying them on a terminal in response to your commands. This usage often confuses mundanes. [Jargon File]