Contraction of I would not or I should not.
I'd (I would/should) + n't (not)
I'dn't want that here.
I'dn't be there.
To have not done something; an abbreviation of "I had not", coming from the abbreviations "I had" (I'd) and "had not" (hadn't).
Pete - "I think you should study chapter 12 for your Humanities exam."
Rachel - "Good idea, I'dn't thought about that chapter."