The Collaborative International Dictionary of English v.0.48:

Laches \Lach"es\ (l[a^]sh"[e^]z), Lache \Lache\ (l[a^]sh), n.
   [OF. lachesse, fr. lache lax, indolent, F. l[^a]che,
   ultimately fr. L. laxus loose, lax. See Lax.] (Law)
   Neglect; negligence; remissness; neglect to do a thing at the
   proper time; especially, a delay in asserting a claim,
   sufficient to cause a person to lose the right to
   adjuducation of the claim by a court.
   [1913 Webster]

         It ill became him to take advantage of such a laches
         with the eagerness of a shrewd attorney. --Macaulay.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

LACHES. This word, derived from the French lecher, is nearly synonymous with
negligence.
     2. In general, when a party has been guilty of laches in enforcing his
right by great delay and lapse of time, this circumstance will at common law
prejudice, and sometimes operate in bar of a remedy which it is
discretionary and not compulsory in the court to afford. In courts of
equity, also delay will generally prejudice. 1 Chit. Pr. 786, and the cases
there cited; 8 Com. Dig. 684; 6 Johns. Ch. R. 360.
     3. But laches may be excused from, ignorance of the party's rights; 2
Mer. R. 362; 2 Ball & Beat. 104; from the obscurity of the transaction; 2
Sch. & Lef. 487; by the pendancy of a suit; 1 Sch. & Lef. 413; and where the
party labors under a legal disability, as insanity, coverture, infancy, and
the like. And no laches can be imputed to the public. 4 Mass. Rep. 522; 3
Serg. & Rawle, 291; 4 Hen. & Munf. 57; 1 Penna. R. 476. Vide 1 Supp. to
Ves. Jr. 436; 2 Id. 170; Dane's Ab. Index, h.t.; 4 Bouv. Inst. n. 3911.