straw man

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A person who is set up as a cover or front for a questionable enterprise.
2. n. An argument or opponent set up so as to be easily refuted or defeated.
3. n. A bundle of straw made into the likeness of a man and often used as a scarecrow.

Wiktionary

1. n. A doll or scarecrow (particularly one stuffed with straw).
2. n. An insubstantial concept, idea, endeavor or argument, particularly one deliberately set up to be weakly supported, so that it can be easily knocked down; especially to impugn the strength of any related thing or idea.
3. n. An innocuous person or someone of nominal or lesser importance, as a front man or straw boss.

WordNet 3.0

1. n. a person used as a cover for some questionable activity
2. n. a weak or sham argument set up to be easily refuted
3. n. an effigy in the shape of a man to frighten birds away from seeds