I always thought it was a church key 'cause it looked like a church window!
Though Wikipedia differs in their oppinion.
"There is sparse, and often contradictory, documentation as to the origin of the term "church key". The phrase is likely a
sarcastic euphemism, as the opener was obviously not designed to access churches. One explanation is in
Medieval Europe, most brewers were
monks.
Lagering cellars in the
monasteries were locked to protect aging beers, and the monks carried keys to these lagering cellars. It may have been those keys, which remotely resembled the early church key openers, that gave the "church key" opener its name.[SUP]
[16][/SUP] Another motive for assigning the device such an ironic name could have been the fact beer was first canned (for test marketing) in 1933[SUP]
[17][/SUP]—the same year
Franklin Delano Roosevelt signed the
Cullen-Harrison Bill.[SUP]
[18][/SUP] This act, which predated
Repeal of Prohibition, amended the
Volstead Act, making
3.2% low-alcohol beer legal. Some experts have posited the term "churchkey" was a way to "stick it to" the religious organizations who had effected
Prohibition in the first place" source Wikipedia.