What's the meaning of "picking up the ashes"?

Pickpocket, a word describing incest, specifically refers to incest in which a father-in-law and daughter-in-law have sexual relations.

Source of the saying:

Wang Youguang of the Qing Dynasty wrote "Wuxia Proverbial Union", which recorded: "Weng private its daughter-in-law, commonly known as picking the gray. Little is known about its meaning. According to a temple in the past, the incense is particularly strong, tin foil acid burning furnace, ash accumulation day more, panned out of its tin, the city got a lot of money. Temple neighbors know, picking up its ash, theft of its tin as often. Pickpocket ash, steal tin also. Tin, daughter-in-law homophonic, as a cryptic language."

Meaning: the temple incense burning furnace, burning more tin foil, over time, the formation of a large block, the monks will pick out to sell money with. Later, the people next to the temple know, also come to the furnace to steal tin. Because "tin" "daughter-in-law" homophonic, it is derived from the old man stole the daughter-in-law of the cryptic.

Expanded:

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The Dream of the Red Mansions

(Cheng A-ben) In the seventh chapter of Dream of the Red Mansions, "Delivering the Palace Flowers", "Jia Lian's Drama to Xifeng", "Feasting at Ningfu", "Bao Yu's Meeting", "Wang Xifeng", and "Jia Bao Yu", "returning to Rongguofu", "returned to Rongguofu", encountered the drunken slave of the Jiafu, Jia Dada, who scolded him, "I'm going to the Ancestral Hall to cry for the Grand Master. Where to expect to now give birth to these animals! Every day the family steals the dog to play chicken, climb the ash of the climb ash, raise the uncle of the raise uncle, what I do not know? We're hiding our arms in our sleeves!" .

Reference:

Baidu Encyclopedia - Graying