Actually, you have that backwards, you see, the act was known as 'shagging', from the old english 'shagadelic' meaning excellent or enjoyable, long before the carpet existed. The style of carpet was found to be ideal for 'shagging' on, especially in groups, as the long fibres reduced slipping. Thus it became known as 'shag pile' due to the piles of 'shaggers' often using them.

This post is brought to you by the letters B, S, and insufficient sleep.