To be in lieu of something is to replace it or substitute for it. A restaurant that's run out of clams might serve French onion soup in lieu of chowder.

The word lieu originally comes from the Latin locus, meaning "place," and its meaning has stayed true to its origins ever since. Though it does have a standalone definition, "the position or function formerly held by another," this noun is most commonly encountered in the phrase "in lieu of," which means, basically, "instead of."

Definitions of lieu
  1. noun
    the post or function properly or customarily occupied or served by another
    “in lieu of”
    synonyms: place, position, stead
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    as the agent of or on someone's part (usually expressed as "on behalf of" rather than "in behalf of")
    type of:
    function, office, part, role
    the actions and activities assigned to or required or expected of a person or group
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