'말발' is a quite interesting word and according to the linked Naver Korean dictionary, it means:
듣는 이로 하여금 그 말을 따르게 할 수 있는 말의 힘.
We often hear "말발이 서다" and "말발을 세우다" and what is the etymology of the word? '말' means word, language, speech, etc. and what does '발' mean in this word?