c. 1300, "wheel having teeth or cogs;" late 14c., "tooth on a wheel," probably a borrowing from a Scandinavian language (compare Norwegian kugg "cog") and cognate with Middle High German kugel "ball."
1777, "a projecting part of a rotating machinery used to impart motion to another part," from Dutch cam "cog of a wheel," originally "comb," from Proto-Germanic *kambaz "comb," from PIE root *gembh- "tooth, nail." It is thus a cognate of English comb (n.). This might have combined with English camber "having a slight arch;" or the whole thing could be from camber. It converts regular rotary motion into irregular, fast-and-slow rotary or reciprocal motion. "The original method was by cogs or teeth fixed or cut at certain points in the circumference or disc of a wheel ..." [OED]. Cam-shaft attested from 1850.
"perceive, become conscious of," 1650s, back-formation from cognizance. The French word is connaître (Old French conoistre). Related: Cognized; cognizing.
mid-15c., cognicioun, "ability to comprehend, mental act or process of knowing," from Latin cognitionem (nominative cognitio) "a getting to know, acquaintance, knowledge," noun of action from past participle stem of cognoscere "to get to know, recognize," from assimilated form of com"together" (see co-) + gnoscere "to know" (from PIE root *gno- "to know"). In 17c. the meaning was extended to include perception and sensation.
"to think earnestly or seriously," 1560s (transitive); 1630s (intransitive); from Latin cogitatus, past participle of cogitare "to think" (see cogitation). Related: Cogitated; cogitating.
"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.