Someone who can't make a decision; either in support of you or someone else. Will whinge and complain in the back end but won't stand up or voice their opinions or views at the front end.
They will never have your back in a verbal or physical stouch and likely to be first out the door when it kicks off.
"He had a chance to speak up and say what was on his mind in that meeting the weak cunt."
"What is it with that weak cunt sneaking off like that?"