Is it ensure, insure, or assure?
"negligence in performance of legal dute," 1570s, earlier simply "slackness, negligence, want of zeal," late 14c., from Anglo-French laches, Old French lachesse, from Old French lasche (Modern French lâche), verbal adj. from lascher, from Vulgar Latin *lascare, classical laxare, from laxus (see loose). Cf. riches.