Meaning of "See that wet, see that dry!"

Question

"You'd share and share alike with the rest, whether you'd been in that particular job or not. There's fifty members, and you'd get one-fiftieth, same as Number One and same as me."
"Really? No kidding?"
"See that wet, see that dry!" Jukes laughed. "Say, can you beat it? There's never been anything like it. It's the biggest thing ever been known. He's a great man, is Number One."
"And do you pull off many jobs?"

The above dialogue is happening between two persons, one is a recruiter of gangster, the other one is would-be gang member.

"See that wet, see that dry!" seems to me 'it needs to be experience yourself.' But in fact I am not very sure of the meaning since it doesn't look like a common expression. Or it might be an invented expression by Ms. Dorothy Sayers herself.

The story was written by Ms. Sayers in 1929 by the name of 'The Adventurous exploit of Cave of Ali Baba.'

Answer 1

It's an assertion that the truth is being told, from what little I've managed to glean just now researching your question.

As the first part of a rhyme asserting the truth here (see 246)

"See that wet, see that dry. Whack my back if I tell a lie.”

and here

"See that wet see that dry cross my heart and hope to die!"

Answer 2

Growing up in England over 60 years ago I remember it as 'See it wet, see it dry, hope to die if I tell a lie'. You had to lick your index finger and then wipe it dry on your clothes, showing the results to the listener to make your point. Funnily enough I had just finished the Lord Peter Wimsey story mentioned and came on this site to see if it was still common parlance as I have not seen it used or spoken in decades.

Answer 3

In this feature film "The Rocking Horse Winner" (1949), based on the short story by D. H. Lawrence around at 29:33 the young Master Paul says to basset : See that's wet, see that's dry, cross my heart if i tell a lie ! to seal their secret partnership. This feature film is on youtube.