Where did the term "church key" for a hand held can opener come from?

The shape of the business end of the tool reminded people of the often ornate handles to big, old-fashioned door keys. The link with churches in particular was surely because in the experience of most people such big keys opened church doors. It’s also more than probable that an irreverent joke was attached as well, in that drinking beer was an unchurchly thing to do.

Before the messages start to be written, let me rapidly move on to a further stage in the development of the term. As you say, the phrase church key is only recorded in print from 1951, though there is much anecdotal evidence to suggest it is rather older in the spoken language, perhaps from the late 1930s. This was around the time at which beer began to be sold in cans rather than bottles. These early cans also needed a tool to open them, since the pull tabs of today were not to be introduced until about 1962. The tool was a stout flat strip of metal with a sharp point, which you pressed into the top of the can to puncture a triangular hole (two were needed on opposite sides, I recall, to let air in so the beer would flow easily). By an obvious analogy, these also came to be called church keys, even though they were a completely different shape.

Church Key Opener

Although some resemble keys and may be kept on key rings, the phrase may just be a sarcastic euphemism, since it is used for opening beer bottles (and old-fashioned beer cans) and not churches. Other explanations give it an almost mythic significance; for example, in Medieval Europe, monks and nobility were the only brewers. Lagering cellars in the monasteries were locked, as the monks guarded the secrets to their craft. The monks carried keys to these lagering cellars on their cinch - or belts. It was this key from which the "Church Key" opener gets its name.