People often say tznius when they mean to say tzanua. Do you know of any other examples in Hebrew, Aramaic, or Yiddish in which people use the noun but they really mean to use the adjective?
Not specifically adjectival use of nouns, but:
- man d'amar, Aram. non-constituent, to mean "opiner", noun
- hava amina, Aram. verb phrase, to mean "first thought", noun
- teku, Aram. verb, to mean "unresolved question", noun
- ma matzinu, Heb. non-constituent (I think), to mean "logical argument from one thing to a comparable thing", noun
- k'le kodesh, Heb. plural noun, to mean "one who works in religion", singular noun (should be k'li kodesh)
- nafka minah, Aram. verb phrase, to mean "result", noun
- upshern, Yid. verb (I think), to mean "haircut", noun
Surely someone will think of more, though.