Remiss

1. At fault; failing to fulfill responsibility, duty, or obligations.
2. Not energetic or exact in duty or business; careless; tardy; slack; hence, lacking earnestness or activity; languid; slow.

Synonyms

• (at fault): at fault, blameworthy, lash, lax, negligent, reprehensible
• (not energetic): careless, tardy, slack, languid, slow; apathetic