egg in your beer
A bonus, something for nothing, as in What do you want—egg in your beer? This expression dates from about 1940 and became widespread during World War II. The origin is unknown, since adding egg to beer does not improve the taste.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.