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"to pledge" (obsolete except in archaic plight one's troth), from Old English pligtan, plihtan "to endanger, imperil, compromise," verb form of pliht (n.) "danger, risk" (see plight (n.2)). Related: Plighted; plighting.
"condition or state (usually bad)," late 12c., "danger, harm, strife," from Anglo-French plit, pleit, Old French pleit, ploit "condition" (13c.), originally "way of folding," from Vulgar Latin *plictum, from Latin plicitum, neuter past participle of Latin plicare "to fold, lay" (see ply (v.1)).
Originally in neutral sense (as in modern French en bon plit "in good condition"), sense of "harmful state" (and current spelling) probably is from convergence and confusion with plight (n.2) via notion of "entangling risk, pledge or promise with great risk to the pledger."
"pledge," mid-13c., "pledge, promise," usually involving risk or loss in default, from Old English pliht "danger, risk, peril, damage," from Proto-Germanic *pleg- (cf. Old Frisian plicht "danger, concern, care," Middle Dutch, Dutch plicht "obligation, duty," Old High German pfliht, German Pflicht "obligation, duty" (see plight (v.)). Cf. Old English plihtere "look-out man at the prow of a ship," plihtlic "perilous, dangerous."