Webster Dictionary (0.00 / 0 votes) Rate this definition: Brim (noun) the rim, border, or upper edge of a cup, dish, or any hollow vessel used for holding anything. Brim (noun) the edge or margin, as of a fountain, or of the water contained in it; the brink; border. Brim (noun) the rim of a hat. Brim (verb) to be full to the brim. Brim (verb)