A prop name or reference, often referring to Asta from 'The Thin Man' films, a fictional terrier; can mean a small dog or pet.
This is primarily a proper name (Asta the dog character), popularized through 1930s-40s Hollywood films. As a common noun, it's informal slang derived from the famous character's name.
Asta the wire-haired terrier was one of the first dog actors in Hollywood, actually upstaging human actors in 'The Thin Man'—so famous that the character's name became generic slang for small dogs in mid-20th century American English, showing how movies shape our vocabulary.
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