It means "no comment."
It came about because Gordon Brown did a webchat when he was PM.
It had become traditional for webchat guests to be asked what their favourite biscuit is. Probably partly because GB has very poor eyesight, most of his answers were written in advance to tge questions posted in advance and cut-and-pasted into the thread (you could tell) and he didn't really respond to the ones asked on the day.
Posters started to repeat the question about biscuits. It was seen as very hilarious but was actually a bit sixth form and rather embarrassing.
The fact that he had refused to answer a question about biscuits made the national press and even Have I Got News For You. So a man whose eyesight was so poor he could see the questions popping was made to look a fool, unfairly, and Mumsnetters who had the opportunity to ask the Prime Minister any number of searching questions but chose instead to focus on biscuits also looked fools- fairly.
GB (more likely his team) popped back to the thread in the following week to say sorry and reveal what biscuits he liked.
MNHQ introduced the biscuit emoticon shortly afterwards to represent "no comment" in a - in my view at any rate - rather feeble attempt to make a joke about what was not really Mumsnet's finest hour.
As a PP has said though it tends to mean "you are such an arse I can't be bothered to reply to you but I want you to know that I can't be bothered."