rea·son·less \ˈrēzənlə̇s\adjective Etymology: Middle English resonles, from reson, resoun reason + -les -less 1. a.: not having the faculty of reason < a reasonless brute > b.: not marked by the use of reason : devoid of common sense : senseless < a reasonless hostility > 2.: not based on or supported by reasons < an apparently arbitrary and reasonless change > • rea·son·less·lyadverb • rea·son·less·nessnoun-es