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1530s, from Latin enormis "out of rule, irregular, shapeless, extraordinary, very large," from ex- "out of" (see ex-) + norma "rule, norm" (see norm), with English -ous substituted for Latin -is.
Meaning "extraordinary in size" is attested from 1540s; original sense of "outrageous" is more clearly preserved in enormity. Earlier in same sense was enormyous (mid-15c.). Related: Enormously.