John Gill Commentary 1 Kings 20:11

"And the king of Israel answered and said, Tell him, Let not him that girdeth on [his armor] boast himself as he that putteth it off." — 1 Kings 20:11 (ASV)

Ver. 11 And the king of Israel answered and said
Very mildly and very wisely:

tell him, let not him that girds on his harness boast himself as he
that puts it off ;
that is, he that prepares for the battle as he that has got the victory; the sense is, let no man triumph before the battle is over and the victory won; the events of war are uncertain; the battle is not always to the strong.