clay pigeon
clay pigeon {n.}, {slang}, {informal}
1. A popular target at practice shooting made of clay and roughly resembling a pigeon; an easy target that doesn't move.
All he can shoot is a clay pigeon.
2. A person who, like a clay pigeon in target practice, is immobilized or is in a sensitive position and is therefore easily criticized or otherwise victimized.
Poor Joe is a clay pigeon.
3. A task easily accomplished like shooting an immobile clay pigeon.
The math exam was a clay pigeon.
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Source: A Dictionary of American Idioms