Definition of tantamount

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tan·ta·mount    (tnt-mount) KEY 

ADJECTIVE:

Equivalent in effect or value: a request tantamount to a demand.

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ETYMOLOGY:
From obsolete tantamount, an equivalent, from Anglo-Norman tant amunter, to amount to as much : tant, so much, so great (from Latin tantum, neuter of tantus, from tam, so; see to- in Indo-European roots) + amunter, to amount to variant of Old French amonter ; see amount