WordNet sense 1 (not showing due care or attention):

inattentive, neglectful

WordNet sense 2 (lacking in rigor or strictness):

lax, slack

WordNet sense 3 (failing in what duty requires):

delinquent, derelict, remiss

WordNet sense 4 (marked by lack of attention or consideration or forethought or thoroughness; not careful):

careless