I agree that
conchie should not be common, and I'll change it to rare.
But I was slightly surprised by MK's surprise, because the word has another usage, which I had assumed was from Britain originally. In Australia, to be
conchie is to be overconscientious, and a
conchie is an overconscientious person. It turns out this usage is restricted to Australia. The word, in these senses, is mildly derogatory. I assume the reason we have such a term is not that there are a lot of conscientious people here, but rather that we Australians are the only people in the world who feel that being conscientious is deserving of criticism!
This Australian term is not so old. The
Australian National Dictionary gives
conshie as the primary spelling, and lists a few other variants. Its earliest citation is from Alex Buzo in 1969: "You're a bit of a conch this morning, aren't you?" Its earliest examples for
conchie are two from 1980, both putting the word in quotes. An earlier example than any in the
AND is from the 1966 2nd edition of
The Australian Language, by Sidney J Baker, where
conchie is listed as a word in use among Australian teenagers.
This word has been discussed here before. In
December 2011, in looking at a recent puzzle that was thought to be unusually difficult, I noted that
conchie had been played by only about ten percent of players, making it the most elusive of "common" words that day. In the rest of the discussion, a few people suggested conchie should be rare, and no-one disagreed, but I didn't follow up on it. (Not conchie!)