It takes about 2 seconds to check anything online these days.

BVD is a brand of men's underwear, which are commonly referred to as "BVDs."

BVD stands for Bradley, Voorhees & Day, the New York City firm that initially manufactured underwear of this name for both men and women. It was founded in 1876 and named for its three founders.

BVD first manufactured bustles for women. They then became famous for their men's union suits made of heavy knitted fabric. In 1908, that bulky and tight fitting garment was turned into a new kind of loose fitting underwear. They went on to introduce a two-piece and the popular union suit.

They introduced a lightweight waffle-like fabric with the advertising slogan, "Next to Myself I Like BVD Best."

http://www.answers.com/topic/bvd

It is quite obviously the Union suits made of heavy woollen fabric which originally made the brand famous which are being referenced in the lyric, the meaning being that it would take a lot to get through the knees of such a heavy fabric.

Raise your game, people.