mid-15c., "of unnatural formation, deviating from the natural order, hideous," a variant of earlier monstruous (late 14c.), from Old French monstruos, monstrueuse and directly from Late Latin monstruosus "strange, unnatural, monstrous," from Latin monstrum "divine omen, portent, sign; abnormal shape; monster, monstrosity," figuratively "repulsive character, object of dread, awful deed, abomination," from root of monere "to admonish, warn, advise," from PIE *moneie- "to make think of, remind," suffixed (causative) form of root *men- (1) "to think."
Meaning "enormous, huge" is from c. 1500; that of "outrageously wrong, shocking, horrible" is from 1570s. The earlier form monstruous remained "very common in the 16th c." [OED]. As an adverb from c. 1600, but by late 19c. regarded as vulgar or colloquial. Related: Monstrously; monstrousness.
"of the nature of a portent, ominous," 1540s, from Latin portentosus "monstrous, marvelous, threatening," from portentem "portent," from portendere (see portend). Sometimes portentious, by influence of pretentious. Related: Portentously.
Cabalistic word associated with the followers of Basilides the Gnostic, by 1680s, of uncertain origin and with many elaborate explanations. Also used in reference to a type of Gnostic amulet featuring a carved gem depicting a monstrous figure and obscure words or words connected to Hebrew or Egyptian religion (1725).
monstrous predatory bird of Arabian mythology, 1570s, from Arabic rukhkh, from Persian rukh. It is mentioned in Marco Polo's account of Madagascar; according to OED, modern use of the word mostly is due to translations of the "Arabian Nights" tales. Hence roc's egg "something marvelous or prodigious." Compare simurgh.
"distorted projection or drawing" (one that looks normal from a particular angle or with a certain mirror), 1727, from Greek anamorphosis "transformation," noun of action from anamorphoein "to transform," from ana "up" (see ana-) + morphosis, from morphe "form," a word of uncertain etymology. In botany, "monstrous development of a part" (1830); in evolutionary biology, "gradual change of form in a species over time" (1852).