LACK' The term 'weakness in an argument' can be summarized by the four-letter word 'LACK'. A LACK in an argument is when there is a deficiency or absence of evidence to support the argument. This could be due to a LACK of facts, data, or other forms of proof that would make the argument more convincing. It could also be due to a LACK of logical reasoning or structure in the argument. In either case, a LACK in an argument can weaken its overall strength and validity.