"compelling assent or conviction," 1650s, from French cogent "necessary, urgent" (14c.), from Latin cogentem (nominative cogens), present participle of cogere "to curdle; to compel; to collect," literally "to drive together," from assimilated form of com "together" (see co-) + agere "to set in motion, drive, drive forward; to do, perform" (from PIE root *ag- "to drive, draw out or forth, move"). Related: Cogently.

Proto-Indo-European root meaning "to drive, draw out or forth, move."

It forms all or part of: act; action; active; actor; actual; actuary; actuate; agency; agenda; agent; agile; agitation; agony; ambagious; ambassador; ambiguous; anagogical; antagonize; apagoge; assay; Auriga; auto-da-fe; axiom; cache; castigate; coagulate; cogent; cogitation; counteract; demagogue; embassy; epact; essay; exact; exacta; examine; exigency; exiguous; fumigation; glucagon; hypnagogic; interact; intransigent; isagoge; litigate; litigation; mitigate; mystagogue; navigate; objurgate; pedagogue; plutogogue; prodigal; protagonist; purge; react; redact; retroactive; squat; strategy; synagogue; transact; transaction; variegate.

It is the hypothetical source of/evidence for its existence is provided by: Greek agein "to lead, guide, drive, carry off," agon "assembly, contest in the games," agōgos "leader," axios "worth, worthy, weighing as much;" Sanskrit ajati "drives," ajirah "moving, active;" Latin actus "a doing; a driving, impulse, a setting in motion; a part in a play;" agere "to set in motion, drive, drive forward," hence "to do, perform," agilis "nimble, quick;" Old Norse aka "to drive;" Middle Irish ag "battle."

"substance that produces coagulation," 1770, from Latin coagulantem (nominative coagulans), present participle of coagulare "cause to curdle" (see coagulate).

c. 1400, coagulacioun, "act of changing from a fluid to a thickened state," from Latin coagulationem (nominative coagulatio), noun of action from past participle stem of coagulare "cause to curdle" (see coagulate). Meaning "mass or quantity of coagulated matter" is from 1660s.

c. 1400, "have a morbid craving;" early 15c., "grow feeble or sick, begin to die;" mid-15c., "to fade, fail, give way," probably from Middle Dutch quelen "to suffer, be ill," from Proto-Germanic *kwaljan (source also of Old Saxon quelan "to die," Old High German quelan "die," German quälen "suffer pain"), from PIE root *gwele- "to throw, reach," with extended sense "to pierce."

Or perhaps from obsolete quail "to curdle" (late 14c.), from Old French coailler, from Latin coagulare (see coagulate).

Sense of "lose heart or courage, shrink before a danger or difficulty, cower" is attested from 1550s. According to OED, the word was common 1520-1650, then rare until 19c., when apparently it was revived by Scott. Related: Quailed; quailing.