Unknown. Suspected to be an old fishermen’s term from Ancient Greek ἁλμῡρῐ́ς (halmūrĭ́s, literally “saltiness”) which, as well as ἅλμη (hálmē), also meant a kind of fish-sauce or brine. Or, from Proto-Indo-European *(s)mewk- (“damp, slimy, slippery”), see also mucus.