The usage of the word for a disease found in birds appears to be unique to Anglo-Norman. Also the substantival use is not attested elsewhere, but its sole citation may be a corruption of [goute] pocrous. The monophtongisation of ‘oa’ to ‘o’ may be the result of an association of the word with Middle English pok n. (‘skin eruption, sore, pustule, pimple etc.’), even though such an adjective is not attested in English.