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mid-15c., "unnatural, deviating from the natural order, hideous," from Middle French monstrueux, from Latin monstruosus "strange, unnatural, monstrous," from monstrum (see monster). Meaning "enormous" is from c.1500; that of "outrageously wrong" is from 1570s. Earlier form monstruous (late 14c., from Old French monstruous) was "very common in the 16th c." [OED].