remiss

Meaning careless about a duty; failing to do what you should

I would be remiss if I didn’t thank the volunteers who set up all the chairs.

In context The coach admitted that he had been remiss: he had forgotten to check the team’s helmets before the game, and two of them turned out to be cracked.

Similar neglectful, negligent, lax, careless

Opposite careful, attentive, diligent

From Latin remissus, “slack, relaxed,” from remittere, “to send back, to let go, to ease off.”