late 14c., disavauntage, "loss, injury, prejudice to interest," from Old French desavantage (13c.), from des- "not, opposite of" (see dis-) + avantage "advantage, profit, superiority" (see advantage). Meaning "that which prevents success or renders it difficult" is from 1520s.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
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Definitions of disadvantageous from WordNet
involving or creating circumstances detrimental to success or effectiveness;
a well-known study from the 1970's showed that gender stereotyping placed women in a disadvantageous position