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Entry 6 senses · 2 variants Webster, 1913

Rat

/(răt)/ · IPA /ɹæt/
01 n. One of several species of small rodents of the genus Rattus (formerly included in Mus) and allied genera, of the family Muridae, distinguis…
  1. 1.
    One of several species of small rodents of the genus Rattus (formerly included in Mus) and allied genera, of the family Muridae, distinguished from mice primarily by being larger. They infest houses, stores, and ships, especially the Norway rat, also called brown rat, (Rattus norvegicus formerly Mus decumanus), the black rat (Rattus rattus formerly Mus rattus), and the roof rat (formerly Mus Alexandrinus, now included in Rattus rattus). These were introduced into America from the Old World. The white rat used most commonly in laboratories is primarily a strain derived from Rattus rattus.(Zool.)
  2. 2.
    A round and tapering mass of hair, or similar material, used by women to support the puffs and rolls of their natural hair.[Local, U.S.]
  3. 3.
    One who deserts his party or associates; hence, in the trades, one who works for lower wages than those prescribed by a trades union.[Cant]
Phrases & compounds
Bamboo rat — any Indian rodent of the genus Rhizomys.
Beaver rat — See under Beaver and Coast.
Blind rat — the mole rat.
Cotton rat — a long-haired rat (Sigmodon hispidus), native of the Southern United States and Mexico. It makes its nest of cotton and is often injurious to the crop.
Ground rat — See Ground Pig, under Ground.
Hedgehog rat — See under Hedgehog.
Kangaroo rat — the potoroo.
Norway rat — the common brown rat. See Rat.
Pouched rat — See Pocket Gopher, under Pocket.
Rat Indians — a tribe of Indians dwelling near Fort Ukon, Alaska. They belong to the Athabascan stock.
Rat mole — See Mole rat, under Mole.
Rat pit — an inclosed space into which rats are put to be killed by a dog for sport.
Rat snake — a large colubrine snake (Ptyas mucosus) very common in India and Ceylon. It enters dwellings, and destroys rats, chickens, etc.
Spiny rat — any South American rodent of the genus Echinomys.
To smell a rat — See under Smell.
Wood rat — any American rat of the genus Neotoma, especially Neotoma Floridana, common in the Southern United States. Its feet and belly are white.
02 v. i. In English politics, to desert one's party from interested motives; to forsake one's associates for one's own advantage; in the trades, to …
imp. & p. p. Ratted; p. pr. & vb. n. Ratting
  1. 1.
    In English politics, to desert one's party from interested motives; to forsake one's associates for one's own advantage; in the trades, to work for less wages, or on other conditions, than those established by a trades union.
    “Coleridge . . . incurred the reproach of having ratted, solely by his inability to follow the friends of his early days.” — De Quincey.
  2. 2.
    To catch or kill rats.
  3. 3.
    To be an informer (against an associate); to inform (on an associate); to squeal; -- used commonly in the phrase to rat on.