A disarming smile, question, or bit of honesty makes you feel less angry, less ready to judge, and a little more friendly. Nobody has taken a gun from your hands; they have taken the fight out of you. It is almost always a compliment: you did not mean to like them, and then you did. People use it of charm, of a joke that arrives at the right second, or of someone who admits a fault so plainly that you cannot keep scolding. Though actually, it can also make you a little uneasy, because you can feel yourself being won over. In careful writing the old military picture is still there — to disarm is to take weapons away — but this everyday sense is about lowering your guard, not about armies.