Actually I don't know what exactly it means, but there is a similar idiom in my mother tounge so maybe I can take a guess. I think it is an idiom that used when the thing you are looking for is near you or so close to you. Like looking for a pen on the table while it is in your pocket or looking for glasses while wearing them. So, if they were snakes, they would have bit you :D

My grandma used to say, "If it was a bear it wouldda bit you."  She usually said this after we found something that was right under our noses the whole time, such as keys or glasses or cooking tools, especially if we had to ask her where the thing was.

In Portuguese we use it a lot. For example, that moment when you're looking for an object but you can't find this, even if the object is really near. Then, when we find it, we say: If it was a snake, it would bit me - due the distraction.
Also we can say that the object is "below of the nose" to represent the same situation.

I could quote idioms until the cows come home. Put a fork in me, I'm done. 

Thanks for your correction, Daniel.

In Portuguese it is: abaixo do nariz.