The big 'I am'
Someone thinking they're all that;When one thinks they're of higher authority/importance than they are; acting like the legend only you believe yourself to be!
Did you see that bloke? Giving it all that, thinking he's the big 'I am'?!
or
"So I'm there, giving it the big 'I am', and then I realize I've got nothing to back it up with!".
or
"So I'm there, giving it the big 'I am', and then I realize I've got nothing to back it up with!".