What is the precise meaning of letting one's garment hang down below the ankles (isbal al-izar), and does it apply to someone who constantly wears a suit and cannot noticeably shorten the trousers?

Al-isbal is the lengthening and letting down of garments below the ankles. It is forbidden based on the Prophet's (peace and blessings be upon him) saying: "Whatever is below the ankles of the izar (garment) is in the Fire." This means that the part of the foot of the wearer of the trailing izar that is below the ankles will enter the Fire as a punishment for his action.