In English, you would normally say “I know what’s best for you” not “better for you”
French sentence: Je sais pas.
English Translation
I haven’t got a clue.
I’ve always been wondering, but haven’t dared to ask: whenever I hear French people saying “I don’t know” in films, they seem to be saying “J’ai pas”, not “Je sais pas”. The same goes for “I’m not”; I hear “J’uis pas” instead of “Je suis pas”.
Is that just my imagination?
Not just your imagination 
1.Je ne sais pas
2.Je sais pas
3.J’sais pas
4.Chais pas → \ʃe pɑ\
1.Je ne suis pas
2.Je suis pas
3.J’suis pas
4.Chuis pas → \ʃɥi pɑ\
I think you’re hearing #4 in these lists.
ʃ = “ch” sound, in “chercher” (or “sure” in english), that’s the main change here.
It is very common in spoken French.
On a side note, the sentence you are commenting on does not seem to match the thread here.
I think we may have another case of: Sentence comments end up in wrong thread
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Brilliant! Come to think of it, I take it’s of no importance whether the sentence is negated or not; the important thing is that /ʒəs/ becomes /ʃ/ in “je sais” and “je suis”.
I can’t recall ever seeing that in a textbook, so many thanks, @yottapolyglot !
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You’re welcome !
The switch from /ʒəs/ to /ʃ/ is definitely the main thing.
It can be used in both declarative and negative sentences, but I think you are more likely to hear this in negative sentences, especially for “je sais” vs. “je sais pas”.
Je suis pas → Chuis pas: very common in spoken French
Je sais pas → Chais pas: also very common
Je suis → Chuis: I would say this is significantly less common, but I still hear it every now and then.
Je sais → Chais: Even less common (rare). I can’t think of too many examples.
Maybe if someone tells you something you already know, you could reply: Oui je sais → \wi ʃe\
That’s in my experience anyway…
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