This term is a combination of two concepts. The first is the wanking motion that people make when something is over the top (a more explicit version of rolling one's eyes). The second is a physiological phenomenon that occurs when a male wanks off too fast, and the result is thick, and somewhat rubbery. The latter phenomenon implies that the response to the over-the-top nature of the person eliciting the response is very powerful.
Strangers with this kind of honesty make me go a big rubbery one, if you know what I mean.