Old English 1st and 3rd person singular present indicative of cunnan "to know," less commonly as an auxiliary, "have power to, to be able," (also "have carnal knowledge"), from Proto-Germanic *kunnjanan "be mentally able, have learned" (source also of Old Norse kenna "become acquainted, try," Old Frisian kanna "recognize, admit, know," German kennen "know," Middle Dutch kennen "know," Gothic kannjan "make known"), from PIE root *gno- "to know."
It holds now only the third sense of "know," that of "know how to do something" (as opposed to "know as a fact" and "be acquainted with" something or someone). Also used in the sense of may, denoting mere permission. An Old English preterite-present verb, its original past participle, couth, survived only in negation (see uncouth), but compare could. The present participle has spun off with a deflected sense as cunning.
generally, "a small cylindrical sheet-metal vessel used to contain liquids, preserves, etc.," Old English canne "a cup, container," from Proto-Germanic *kanna (source also of Old Saxon, Old Norse, Swedish kanna "a can, tankard, mug," also a unit of measure, Middle Dutch kanne, Dutch kan, Old High German channa, German Kanne). Probably it is an early borrowing from Late Latin canna "container, vessel," from Latin canna "reed," also "reed pipe, small boat;" but the sense evolution is difficult.
The modern sense of "air-tight vessel of tinned iron" is from 1867. The slang meaning "toilet" is c. 1900, said to be a shortening of piss-can; the meaning "buttocks" is from c. 1910, perhaps extended from this.
"5-gallon metal container," 1943, from Jerry "a German." It was first used by German troops in World War II and later adopted by the Allies.
"confident of performance," by 1952, from expression can do "it is possible" (1903), literally "(I or we) can do (it)," which is perhaps based on earlier no can do (see no).
"to study, get to know, peruse carefully," c. 1200, cunnen, "make an attempt, try or seek to do," from Old English cunnian "to know" (see can (v.1)). Related: Conned; conning.