IPA: /ˈflætˌwɜrk/
KK: /flætˌwɜrk/
Laundry items like sheets and linens that can be ironed using a machine instead of by hand.
The hotel sent out all the flatwork to be cleaned and pressed.
Flatwork is formed from "flat" (from Old English *flaet*, meaning level or smooth) and "work" (from Old English *weorc*, meaning activity or labor). The term refers to work done on a flat surface, often in construction or landscaping.
Think of a 'level surface' ('flat') where 'activity' ('work') is performed — that's what flatwork means.