The word "BS" still isn't in dictionaries. "Slang?" Nonsense, that's someone's subjective snobbish judgemental sentiment, about a word that's probably the most often-used noun in the English language.
For that matter, we say:
I don't
We don't
You (singular) don't
You (plural) don't
They don't
but...
He doesn't ?

That's silly.
We traded "Thou" (nominative singular) and "Ye" (nominative plural) for "You" (former accusative plural, now nominative & acc. sing. & pl.). And so now we have to find awkward artificial ways to express you plural, like "You all", "Yous", "You guys", or "You lot". Of course "You guys" is the most common here, except in the South, even when speaking to a group of women. I like "You lot", but someone once asked, "In what part of Florida do they say "You lot"? ".
Bring back Thou and Ye.
And what if you mean "Am I not ...?". But maybe you're in company such that you don't want to sound Shakespearean. Then most people say "Aren't I?" Abyssmal grammar. "I are?" But there's a contraction for "Am not" . It's "Ain't". So you can say, "Ain't I?" But no, that's a no-no 
Michael829
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Michael829