to criticize, especially constantly or repeatedly, in a peevish or petty way; carp: to niggle about the fine points of interpretation; preferring to niggle rather than take steps to correct a situation.
to spend too much time and effort on inconsequential details: It's difficult to be meticulous and not niggle.
to work ineffectively; trifle: to niggle with an uninteresting task.
(intransitive) to find fault continually
(intransitive) to be preoccupied with details; fuss