"tangled and lying flat" (of hair, vegetation, fibers, etc.), 1610s, past-participle adjective from mat (v.).
Entries linking to matted
mat v.
early 15c., matten, "to make mats," from mat (n.1). From 1540s as "to provide with mats, to cover with mats;" meaning "to become tangled" is from 1570s. Related: Matted; matting.