If I were to guess I'd say it is from collecting guano to use as fertiliser. This practice is quite common and used to be so common that there is an expression

If I were to guess I'd say it is from collecting guano to use as fertiliser. This practice is quite common and used to be so common that there is an expression in Norwegian for something selling very well, "Det selger som hakka møkk" [1], which means "It is selling like chopped shit". [1] https://www.sprakradet.no/svardatabase/sporsmal-og-svar/som-hakka-mokk/ https://www.sprakradet.no/svardatabase/sporsmal-og-svar/som-... (Norwegian)