Reconstruction:Proto-Brythonic/pɨskọd

Etymology
Borrowed from.

The classical usage of primarily verbal refers to the act of fishing, but as a verbal noun it developed the sense "catch, caught fish" in a collective sense. This is evidently the meaning that was carried into Proto-Brythonic, where it may have further developed a sense of "fish used as food" continued in Welsh (compare 🇨🇬 for a parallel development). This plural would then have taken a regular singulative form.

Subsequently, was generalized as the plural of  due to the appropriate grammatical number and semantics of the two terms. As a corollary, this resulted in two competing singular terms for "fish", and ; the former was generalized in Breton and Cornish, while the latter became the general form in Welsh.

Noun

 * 1)  food fish, fish meat used as food
 * 1)  food fish, fish meat used as food