mid-14c., coitidian, "daily, occurring or returning daily," from Old French cotidiien (Modern French quotidien), from Latin cottidianus, quotidianus "daily," from Latin quotus "how many? which in order or number?" (from PIE root *kwo-, stem of relative and interrogative pronouns) + dies "day" (from PIE root *dyeu- "to shine").
The qu- spelling in English dates from 16c. Meaning "ordinary, commonplace, trivial" is from mid-15c. Quotidian fever "intermittent fever" is from late 14c. The noun meaning "something that returns or is expected every day" is from c. 1400, originally of fevers.
updated on March 23, 2021