Meaning

noun dust consisting of a mixture of small coal fragments and coal dust and dirt that sifts out when coal is passed over a sieve

noun a noticeable deterioration in performance or quality

noun a stretch of water without current or movement

origin* As an English and Dutch surname, from the adjective slack. * Also as an English surname, from the noun slack (“shallow valley”). * Possibly also a Slovene surname Americanized from slak (“bindweed”), from Proto-Slavic *sъvolkъ. * The name of the software is an acronym of Searchable Log of All Conversation and Knowledge.

Etymology from Wiktionary, CC BY-SA 3.0