A rat is someone who rats. Rats are often found while playing cards (especially Texas Hold'em). Rats rat by catching lucky cards when they make horribler calls. Rats can also be found outside of a card game. Basically when someone does something that is bad for you, you have been ratted.
Non-Rat: I'm all in
(the Non-Rat has pocket Aces. the rat called with 8-9 suited and ratted a flush on the river)
Non-Rat: You fucking rat!!
(if the person who got ratted is Ross, everyone present in the room proceeds to snap and say "yeahhhh" for a at least 10 seconds)
In the wild they live in family groups, much like wolves, and have a similar social structure with an alpha, beta and omega.
The pet rat is the species rattus norvegicus, the brown or norway rat, whereas the rat associated (incorrectly, it must be said) with the plague is rattus rattus, the black rat. Domestic rats do not carry any diseases transmittable to humans and even wild rats are generally cleaner than most other wild animals, as well as the notable fact that it is impossible for a rat to transmit rabies to a person.
Rats have a very undeserved reputation, but this is slowly changing as more people realise what good pets they make.
There is a rat in the garden
Guy 2, "He is such a rat!"
Guy 3, "Wow, what a rat."
Girl 1, "Rat."
Guy 4, "You are all rats!"