late 13c., dismaien, from Old French *desmaier (attested only in past participle dismaye), from Latin de- intensive prefix + Old French esmaier "to trouble, disturb," from Vulgar Latin *exmagare "divest of power or ability" (source of Italian smagare "to weaken, dismay, discourage"), from ex- (see ex-) + Germanic stem *mag- "power, ability" (cf. Old High German magen "to be powerful or able;" see may (v.)). Spanish desmayer "to be dispirited" is a loan word from Old French. Related: Dismayed; dismaying.
c.1300, from dismay (v.).