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Webster 1913 Edition

Noun.

A duck used to lure wild ducks into a decoy; hence, a person employed to lure others into danger.

Beau. & Fl.

Noun.

A duck employed to draw others into a net or situation to be taken.

decoy-duck ‎(plural decoy-ducks)

A duck used to lure wild ducks into a decoy.
(by extension) One employed to lure others into danger.
(Can we find and add a quotation of Beaumont and Fletcher to this entry?)