What is 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 called?

tantamount — 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. (Webster's 1913.)