colloquial abbreviation of certainty, attested by 1889 (in dead cert). Cert (adv.) "forsooth, indeed," was in Middle English, from Old French, from Latin certo, certe, but it became obsolete or dialectal.
now obsolete, but a common oath in the Middle Ages and after, mid-14c., a corruption of the name of the Virgin Mary. It could mean "indeed, forsooth," be a term of asseveration, or be used to express surprise or any other feeling.
"to say, say as follows," from Middle English quoth, from Old English cweþ (Mercian), cwæþ (Northumbrian), third person singular past tense of cweoþan, cweoþa "to say, speak; name, call; declare, proclaim" (Middle English quethan), from Proto-Germanic *kwethanan (source also of Old Saxon quethan, Old Norse kveða, Old Frisian quetha, Old High German quedan, Gothic qiþan).
This is often traced to PIE root *gwet- "to say, speak," but Boutkan, on the grounds of formal objection to proposed cognates (Sanskrit gadati "speaks," Old Welsh guetid "say," Latin vetare "not allow"), has it as of "no (certain) IE etymology," and writes, "This is complicated."
Related to bequeath and bequest. Compare also archaic interjection quotha "forsooth, indeed," originally "said he," 1510s in sarcastic use, "originally a parenthetical phrase used in repeating the words of another with more or less contempt or disdain" [Century Dictionary], from Middle English, from Old English cwæðe ge.