Dairine's "brown eye" comment

[anotherBob]

In the chapter Save and Exit, when Dairine was debating with Logo/LP, she tells him, "You're so full of it that if you had eyes, they'd be brown."

What is that supposed to mean and where did it come from? I am not insulted by it, I just wonder where the phrase comes from.

Bob

[Nights Mistress]

You guys are thinking about it too hard.

She's saying "you're full of crap".

[anotherBob]

I know she was saying he was full of crap, I am just wondering where the saying came from.

Bob