This is a reference to trying to walk straight (walk a line) and pass a field sobriety test, while drunk or intoxicated, in front of a Law Enforcement Officer.
A man passes a field sobriety test while drunk by walking in a straight line. By walking straight, he is released and free to go, even though he broke the law. He fooled the cop and does not have to go to jail.
Buy a
walking straight
mug!
