the.com/cool customer

means Someone who stays calm, detached, and self-possessed under pressure — unbothered when the moment calls for panic.

from 'Customer' has been British slang since roughly the 1600s–1800s for 'a person to be dealt with' — you'll see it in old phrases like 'an awkward customer' or 'a rum customer,' where someone's being sized up like a stranger walking into a shop. 'Cool' supplied its familiar sense of emotional composure, a meaning already well established by the 1700s. Stitch the two together and you get someone so composed they seem to be conducting the transaction of their own crisis without breaking a sweat.