Disarming means so charming that it takes away any wish to be hostile. You arrived ready to complain and now you cannot.
The image is exact, since somebody disarming has removed your weapons without you noticing. It describes smiles, honesty and frankness, and disarming honesty is the most common phrase, because admitting a fault immediately is the fastest way to defuse a criticism. The verb disarm has a literal military sense about removing weapons, and disarmament is the international version of the same thing.
1adjectivedisarmingCharming in a way that removes hostility or suspicion.
“He apologised with disarming honesty.”
Anglo-Norman
desarmer
Middle English
desarmen — to divest of arms
English
disarm
English, today
disarming
disarm comes from Middle English desarmen (“to divest of arms”), from Anglo-Norman desarmer.
Etymology from Wiktionary, CC BY-SA