you ain’t shit - shit still functions as an adjective, in some contexts this might be a good thing, but the phrase “you ain’t ____” most often is used to say the person doesn’t reach the level of the blank. For example “you ain’t all that” means you think/act like you are “all that” but you’re not at the level of “all that” you’re less than all that. If you “ain’t shit” it means you’re so bad that you’re less than shit, you dont even reach the level of shit with how bad you are. This is a devestating insult.
you are not the shit - the shit is a noun, its good, so not being the shit is insulting
you are the shit - the shit is a noun meaning good so this is a complement
shit - adjective, bad
the shit - noun, good
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you are shit - shit is an adjective, you are bad
you ain’t shit - shit still functions as an adjective, in some contexts this might be a good thing, but the phrase “you ain’t ____” most often is used to say the person doesn’t reach the level of the blank. For example “you ain’t all that” means you think/act like you are “all that” but you’re not at the level of “all that” you’re less than all that. If you “ain’t shit” it means you’re so bad that you’re less than shit, you dont even reach the level of shit with how bad you are. This is a devestating insult.
you are not the shit - the shit is a noun, its good, so not being the shit is insulting
you are the shit - the shit is a noun meaning good so this is a complement