The phrase "不了了之" is formed by the negation "不" (not) modifying the verb "了" (to finish) to create "不了" (unfinished), which is then followed by a repetition of "了" combined with the object pronoun "之" (it) to form "了之" (finish it), resulting in a literal sequence meaning "not finished, then finish it", which idiomatically conveys leaving matter unresolved.