Slack Fold Lane

This made it to the top 10 list of rude street names in the Greater Manchester Area, and it does make me laugh. Apparently, a fold is a small hill - and the lane it signposts does gently rise up a... well, I wouldn't even call it a hill, but it does go uphill for a bit. But the slack? Why would a small hill be slack? Its almost as funny as the name of the street I took the picture from - Plodder Lane. Which is more funny because I'm usually crawling along in very heavy traffic. You could say I plod along...