The word 'tantamount' entered English in the mid-17th century from the Anglo-Norman phrase tant amunter, meaning "to amount to as much." This phrase itself derives from Old French tant (so much, from Latin tantus) and amunter (to amount, from Latin ad montem, "to the mountain," suggesting rising to an equal level). The original sense was of one thing rising to the level or value of another. Over time, the phrase fused into a single adjective, shedding its verbal nature to describe a state of complete equivalence, where one action or statement is considered as significant or serious as another.

The word is a simplex loanword from Anglo-Norman. It is not productively formed from separable English morphemes. Its components are historically from Old French tant amunter, meaning "to amount to as much."