remiss
Meaning: negligent, failing to take care
Usage: The burglar gained entrance because the security guard, remiss in his duties, forgot to lock the door.
negligent
Meaning: habitually careless, neglectful
Usage: Jessie’s grandfather called me a negligent fool after I left the door to his apartment unlocked even though there had been a recent string of robberies.
assiduous
Meaning: hard-working, diligent
Usage: The construction workers erected the skyscraper during two years of assiduous labor.
diligent
Meaning: showing care in doing one’s work
Usage: The diligent researcher made sure to check her measurements multiple times.