It is usually said as a sort of challenge. Below is an emotional discussion example between imaginary coworkers Bill and Jim "I don't like my boss. He is incredibly stupid" - Bill "I don't like him either but what can we do." - Jim "Maybe I'll just punch him in the face and be done with it." - Bill "You wouldn't do that!" - Jim "I might just!" -Bill In that example Bill uses "I might just!" as a way of saying he might punch his boss in the face. That phrase doesn't always mean the person will do what they say or that they won't, it really depends on the person. Also the situation doesn't have to be bad it can be anything, emotional or otherwise. Here is a happier example below between two friends "I need to buy new clothes, mine are a little out of style." - Chris "There is a new shop opening down the street, I've heard they have good discounts. You should go there." - Jamie "I might just do that." - Chris I hope those examples helped! Keep learning :-)

I think it may means "He/she has a thought of do it, but they ultimately decided not to do it"