Never heard of it — in a bookish context at least. The word can also be rendered as spue. But, as The Collation informs us, spew apparently results from a “reaction that occurs in some leathers which have been treated (or ‘dressed’) with different fatty or waxy compounds. When the leather is exposed to a change in climate, these unbound compounds migrate towards the surface and solidify, where they form a crystalline residue.” Spew can be found on your old boots as well as your leather-bound library.

It can be distinguished form its dangerous Doppelgänger, mold, by the fact that spew will melt when lightly heated.

The OED informs us that spew may also mean “the fourth swarm of bees in a season”. Good to know we have a word for that.