slang vs drop-dead

slang is a noun but drop-dead is not a noun.

Word	Noun	Adjective	Verb	Adverb
slang	Yes	No	Yes	No
drop-dead	No	No	No	Yes

slang (noun)	drop-dead (noun)
a characteristic language of a particular group (as among thieves)
informal language consisting of words and expressions that are not considered appropriate for formal occasions; often vituperative or vulgar

slang (adverb)	drop-dead (adverb)
	stunningly or breathtakingly