When I think of Quacking, what comes to mind is "If it walks like a duck and Quacks like a duck, its probably a duck."
Lots of things that our (codie alkie druggie mentally ill - insert word of choice) family members or partners say can be pretty predictable based on where they are (or aren't) in their recovery process. So I think of my mentally ill STBXH as a duck. When he says things to me that I react to, I have to process the statement to determine if there is any real value to it or if he is just being a duck. Hence the quacking.