To overdress is to put on clothes that are too formal, too fancy, or too careful for the occasion. A dark suit at a beach picnic is overdressing. A long shiny dress at a casual lunch can be too. The word judges the match between clothes and the day, not how many layers you wear and not how warm you are. It is a verb: don't overdress; she tends to overdress for family dinners. Everyday social English — people use it when they worry about looking out of place. Though actually, 'too formal' is the usual complaint, but looking as if you tried much harder than everyone else can fall under the same word. The opposite idea is to underdress: too casual.