laches (n.)

"negligence in performance of legal duty," 1570s, earlier simply "slackness, negligence, want of zeal" (late 14c.), from Anglo-French laches, Old French lachesse "lawlessness, remissness," from Old French lasche "lax, remiss" (Modern French lâche), verbal adjective from lascher, from Vulgar Latin *lascare, classical laxare "to slacken, relax," from laxus "loose; yielding; indulgent" (from PIE root *sleg- "be slack, be languid"). Compare riches.