any of numerous species of rodents that resembles but is mice but are considerably larger than a mouse and is and are widely regarded as a pests and carriers of disease: genus Rattus and other genera

informal a contemptible person; specif somebody who betrays or deserts friends or associates

informal

NAmer, informal a person who spends a lot of time in a specified place

a street rat

to catch or hunt rats

informal to desert e.g. one's party or cause

informal (+ on) to betray or inform on one's associates

informal (+ on) to default on an agreement