As intensifiers (words that make an adjective stronger), the adjective form of a word (without the ly) is used very often instead of the adverb form in English. As some of the other answers and comments have remarked, these words are indeed adverbs because they modify adjectives.
For example:
bloody stupid (U.K.),
wicked cold (Boston),
dead certain.
If you said bloodily stupid in England, wickedly cold in Boston, or deadly certain pretty much anywhere, it would sound real funny. People say real hot but don't often say real true, because real is an intensifier in the first but not the second. (See Google Ngram).