1510s, "garment consisting of a long piece of woolen cloth, often having a tartan pattern, traditionally worn in Scotland," from Scottish, from or related to Gaelic plaide "blanket, mantle," a word of uncertain etymology, perhaps a contraction of peallaid "sheepskin," from peall "skin," from Latin pellis (but OED finds this "phonetically improbable").
It is a large rectangular piece of woolen stuff, and is worn in Scotland by both sexes for warmth and for protection against the weather. It is a special dress of the Highlanders, and forms part of the uniform of certain infantry regiments in the British army. A variety of the plaid is called maud. [Century Dictionary]
The wearing of it by males was forbidden by Act of Parliament, under penalty of transportation, 1746-82. The meaning "a pattern of bars crossing each other at right angles" is by 1890. As an adjective, "ornamented with a pattern of bars or stripes of color crossing one another at right angles," c. 1600, from the noun.
late 14c., pleit, "a fold, a crease, a flattened gather made by doubling cloth or similar fabric in narrow strips upon itself," also "interlaced strands of hair," from Anglo-French pleit, Old French ploit, earlier pleit, "fold, manner of folding," from Latin plicatus, past participle of plicare "to lay, fold, twist" (from PIE root *plek- "to plait").
late 14c., pleiten, "to fold (something), gather in pleats, double in narrow strips," also "to braid or weave (something)," from plait (n.) and also from Old French pleir "to fold," variant of ploier, ployer "to fold, bend," from Latin plicare "to fold" (from PIE root *plek- "to plait"). Related: Plaited; plaiting.
It forms all or part of: accomplice; application; apply; complex; complexion; complicate; complication; complicity; deploy; display; duplex; duplicate; duplicity; employ; explicate; explicit; exploit; flax; implex; implicate; implication; implicit; imply; multiply; perplex; perplexity; plait; plash (v.2) "to interlace;" pleat; -plex; plexus; pliable; pliant; plie; plight (n.1) "condition or state;" ply (v.1) "work with, use;" ply (v.2) "to bend; ply (n.) "a layer, fold;" replica; replicate; replication; reply; simplex; splay; triplicate.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prasna- "turban;" Greek plekein "to plait, braid, wind, twine," plektos "twisted;" Latin plicare "to lay, fold, twist," plectere (past participle plexus) "to plait, braid, intertwine;" Old Church Slavonic plesti "to braid, plait, twist," Russian plesti; Gothic flahta "braid;" Old Norse fletta, Old High German flehtan "to plait;" Old English fleax "cloth made with flax, linen."
"to interlace, to bend and interweave the branches or twigs of," late 15c. (implied in plashing), from Old French plaissier, from Latin plectere "to plait," from suffixed form of PIE root *plek- "to plait." Related: Plashed.