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c.1400, "lattice, grating," from Old French trelis, originally "sackcloth," from Vulgar Latin *trilicius, from Latin trilicis, genitive of trilix "having three threads, triple-twilled," from tri- three + licium "thread." Cognate with Greek trimitos. Sense extended in Old French to things "woven" of iron, etc., which brought on influence of Old French treille "vine trellis," perhaps from Latin trichila "bower, arbor," which is apparently from Latin triclinium "couch extending round three sides of a table" (for reclining on at meals). Meaning "lattice used to support growing vines" is from 1510s.
framework on which trees and climbing plants are trained. It is usually constructed of long, narrow wood or metal slats that are crisscrossed to produce square or diamond-shaped spaces.