Definition of rat:
(n) :
in scientific usage, any of about 56 different species of small, omnivorous
rodents belonging to the genus Rattus; (colloquial) A term indiscriminately applied to numerous members of several rodent families (e.g.
voles and
mice) having bodies longer than about 12 cm, or 5 inches
"what a rat, leaving us stranded here!"
(v) :
to
betray someone and tell their secret to an authority or an enemy; to turn someone in; to
kill rats
"he ratted on his coworker."