1 a:canid; especially: a highly variable domestic mammal (Canis familiaris) closely related to the gray wolfb: a male dog; also: a male usually carnivorous mammal2 a: a worthless or contemptible personb:fellow, chap<a lazy dog><you lucky dog>3 a: any of various usually simple mechanical devices for holding, gripping, or fastening that consist of a spike, bar, or hookb:andiron4: uncharacteristic or affected stylishness or dignity <put on the dog>5capitalized: either of the constellations Canis Major or Canis Minor6plural:feet7plural:ruin<going to the dogs>8: one inferior of its kind <the movie was a dog>: asa: an investment not worth its priceb: an undesirable piece of merchandise 9: an unattractive person; especially: an unattractive girl or woman10:hot dog 1