"sheet of backing material," 1845, from French mat "dull surface or finish" (15c.), noun use of Old French mat (adj.) "dull, beaten down," for which see mat (adj.). The word has been confused with mat (n.1), especially as the latter was used late 19c. for "piece of thick paper or other material placed for ornament or protection immediately under the glass of a picture-frame, with the central part cut out, for the proper display of the picture." As a verb, "to mount (a print) on a cardboard backing," by 1965. Related: Matted; matting.
also *peik-, Proto-Indo-European root meaning "to cut, mark by incision," hence "embroider, paint."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pimsati "to carve, hew out, cut to measure, adorn;" Greek pikros "bitter, sharp, pointed, piercing, painful," poikilos "spotted, pied, various;" Latin pingere "to embroider, tattoo, paint, picture;" Old Church Slavonic pila "file, saw," pegu "variegated," pisati "to write;" Lithuanian piela "file," piešiu, piešti "to write;" Old High German fehjan "to adorn."
c. 1300, "depicted in a picture;" early 15c., "coated with paint," past-participle adjective from paint (v.). In zoology, used of bright or highly colored creatures; painted-lady is from 1829 as a type of butterfly.