besetting sin
A flaw or shortcoming that someone or something tends to display. I doubt she'll listen to everything you have to say—her lack of patience is her besetting sin.
besetting sin
a fault to which a person or institution is especially prone; a characteristic weakness. The verb beset literally means ‘surround with hostile intent’, so the image is of a sin besieging or pressing in upon a person.
1974 Donal Scannell Mother Knew Best Mother said vanity was a besetting sin which Amy resented, to say the least of it.