remiss

Concise Oxford English Dictionary © 2008 Oxford University Press:
remiss /rɪˈmɪs/

▶adjective lacking care or attention to duty: how very remiss of me, to keep you waiting.

– derivatives
remissly adverb,
remissness noun.

– origin ME (orig. in the senses ‘weakened in colour or consistency’ and (in describing sound) ‘faint’): from L. remissus, remittere ‘slacken’.