Unreasonable delay by a plaintiff in bringing their claim
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laches in Wiktionary, the free dictionary.
In common-law legal systems, laches ( LAT-chiz ; Law French: remissness, dilatoriness, from Old French laschesse) is a lack of diligence and activity in making a legal claim, or moving forward with legal enforcement of a right, particularly in regard to equity. It is an unreasonable delay that can be viewed as prejudicing the opposing party. When asserted in litigation, it is an equity defense, that is, a defense to a claim for an equitable remedy. It is often understood in comparison to a statute of limitations, a statutory defense, which traditionally is a defense to a claim "at law".
The person invoking laches is asserting that an opposing party has "slept on its rights", and that, as a result of this delay, circumstances have changed (witnesses or evidence may have been lost or no longer available, etc.), such that it is no longer a just resolution to grant the plaintiff's claim. Laches is associated with the maxim of equity: "Equity aids the vigilant" - not those who sleep on their rights. Put another way, failure to assert one's rights in a timely manner can result in a claim being barred by laches.