noun

• (zoology) A medium-sized rodent belonging to the genus Rattus.
  "Similar studies of rats have employed four different intracranial resorbable, slow sustained release systems—surgical foam, a thermal gel depot, a microcapsule or biodegradable polymer beads."
• (informal) Any of the numerous members of several rodent families (e.g. voles and mice) that resemble true rats in appearance, usually having short limbs, a pointy snout, a long, hairless tail, and a body length greater than about 12 cm, or 5 inches.
• (informal) A person who is known for betrayal.
  "Near-synonyms: betrayer, scoundrel, snake, snake in the grass, quisling, traitor"
• (informal) An informant or snitch.

verb

• (of a dog, etc.) To hunt or kill rats.
• (intransitive) To betray a political party, cause or principle; to betray someone, to desert a person or thing.
• (informal, intransitive) To work as a scab, going against trade union policies.
• (chiefly US) To backcomb (hair).
  "In 1962, the higher a girl's hair was ratted the more available she was, it was simply understood."

noun

• (regional) A scratch or a score.
• (nautical, regional) A place in the sea with rapid currents and crags where a ship is likely to be torn apart in stormy weather.

verb

• (regional) To scratch or score.
  "He ratted a vertical line on his face with a pocket knife."
• (regional, rare, obsolete) To tear, rip, rend.
  "Ratted to shreds."