can-opener (n.)

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.

"one who or that which opens," Old English openere, agent noun from open (v.). As "a tool or machine used in opening," c. 1600.