mid-13c., "garment, part of a garment," later "side of a building, section of a wall," from Old French pan "section, piece, panel" (11c.), from Latin pannum (nominative pannus) "piece of cloth, garment," possibly from PIE root *pan- "fabric" (cognates: Gothic fana "piece of cloth," Greek penos "web," Old English fanna "flag"). Sense of "window glass" first attested mid-15c.