What is the word used to describe something as being a misnomer?
The adjectival form of the noun 'misnomer' is 'misnomered':
Called by a misnomer, misnamed.
["misnomered, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/253143?rskey=Ib8axh&result=5&isAdvanced=false (accessed March 17, 2016).]
Attestations for the adjective are dated from 1741 through 1989 in OED Online.
The adjective is also listed by American Heritage, among others.
Note that 'to misnomer' is also a verb, one form of which is 'misnomered'. The adjective derived from the noun was used earlier than the verb, appearing first in 1741 as compared to 1795 in the senses given by OED Online for the respective words.
Synonymous adjective options include these:
- misnomed (obsolete)