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Webster 1913 Edition

Noun.

One who bids at an auction in behalf of the auctioneer or owner, for the purpose of running up the price of articles.

[U.S.]

See also: bybidder

by-bidder ‎(plural by-bidders)

(US) One who bids at an auction on behalf of the auctioneer or owner, for the purpose of driving up the price.