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.