寝粉

Etymology
First cited to a text from 1788.

Appears to be a compound of 🇰🇲, from the way that that rice that sits for a long time goes bad.

The sense of "indissoluble grains of dissolved udon flour" has been hypothesized to be a contraction of.

Noun

 * 1)   flour that is too old to be used or eaten
 * 2)  indissoluble grains formed from dissolved udon flour in water (whether boiled or not)