adjectiveEtymology: obsolete tantamount, noun, equivalent, from Anglo-French tant amunter to amount to as much Date: 1641 equivalent in value, significance, or effect <a relationship tantamount to marriage>
Tantamount Tan"ta*mount`, a. [F. tant so much (L. tantus) + E. amount.] Equivalent in value, signification, or effect. A usage nearly tantamount to constitutional right. --Hallam. The certainty that delay, under these circumstances, was tantamount to ruin. --De Quincey.
If you say that one thing is tantamount to a second, more serious thing, you are emphasizing how bad, unacceptable, or unfortunate the first thing is by comparing it to the second. (FORMAL) What Bracey is saying is tantamount to heresy...ADJ: v-link ADJ to n/-ing [emphasis]