The smell of ass even when not presently sniffing one; The lingering effects from placing your nostril on someone's sweaty ass hole and deeply inhaling.
When you and your boss are in a sub/dom relationship, but the roles are reversed and the boss lets you “get one back”, usually involving unprecedented and potentially pain-inducing access to the genital region of the boss. The air exhaled by the boss when they realize it is a mistake is referred to as an Italian sniff.
When I imagine an Italian sniff, I think of Lukaku on his back using Conte's shaven beansack as a punching bag.