To grumble is to murmur or begrudge something. To dispute is a debate, argue or reason through, i.e. leaning on how you understand things.
Paul says, “don’t”. The next verse gives his rationale. That you may prove yourself to be…
The importance of the word ‘prove’ is that it isn’t that you do it to prove to people you are a Christian i.e. “a witness” but rather, this is an assayer’s term meaning to become. It is that the circumstance, that you would normally grumble about, is there to cause you to become blameless and innocent…
Paul used this term because he was aware of the assayers work and the result of his efforts. An assayer would take soil and put it in a crucible. Intense heat would be applied. Impurities would rise to the top and be be skimmed off then it would be allowed to cool down. The process would begin again, seven times, until the dross (impurities) had been eliminated and a perfect bar of silver remains. (Psalm 12:6; 66:10)
Ponder the act of grumbling and how it affects your relationship with others and with God.