Someone who is not all there / someone with mental issues / someone who may be a bit slow / someone who is an idiot

doing anything as fast as you can
Pretending - e.g. "I was just letting on"

no brains
great, excellent, trustworthy e.g. "you'll have no problems with yer man....he's a hunnerd percent so he is"
Bad Luck
To express great dismay at someones cockyness.
God no - e.g. "Gad's no!"