What does For man is a giddy thing mean?

Generally it means we humans are frivolous or impulsive.

It's a quote from Shakespeare's Much Ado About Nothing. Within context of Benedick's speech, he can be translated as saying, "Now that I have decided to marry, I won't take any taunts you may throw at me even though I taunted all of you for getting married. For men are a giddy thing (impulsive) and this is my conclusion."