A dog's dinner it is, then

There are many odd expressions in English that we use unthinkingly, and if you have a mind like mine, you are constantly fretting about their origin. "He's a dab hand at....", for instance. Why "dab"?  I don't know. "Not by a long chalk" - what's chalk got to do with it? And so on.

Some explanations of origins are too bizarre for words. A well-known English expression arose, they say, when Napoleon was at Calais waiting for the weather to improve in readiness for the invasion of England. When the wind dropped and the sea became calm, he turned to his admiral and said "A l'eau, c'est l'heure!"  No way.
But I am taken with the explanation of the origin of one phrase which arose when an orthopaedic surgeon found himself in Soho with a thirst. He went into the nearest tavern, which turned out to be very seedy indeed. As he supped his beer, a painted lady came up and started to proposition him. He ignored her at first, then suddenly took her arm, examined her elbow carefully and said: "What's a nice joint like you doing in a girl like this?"
Take a deep breath, mes potes, I haven't finished yet - not by a long chalk.