ˈtantəˌmau̇nt adjective
Etymology: obsolete English tantamount, n., something equivalent, from tant amount, v., to amount to as much, from Anglo-French tant amunter, from Old French tant so much, as much (from Latin tantum, from neuter of tantus so great, from tam so, so much, as) + amounter to ascend, add up to; akin to Sanskrit tat, neuter demonstrative pron. — more at that , amount
: equivalent in value, significance, or effect
refusal to prolong the truce … would be tantamount to a threat — Current Biography
Synonyms: see same