If you would like to read the above story in Korean, you can find it here.
A long time ago, there is a poor scholar who spends all of his time studying the Chinese classics while his wife struggles alone to make ends meet. One day the scholar returns home unexpectedly from an outing. When he walks into the house, he sees his wife quickly hide something behind her "butt." Thinking that she may be hiding food from him, he demands that she show him what she is hiding. The wife starts crying and says that she is hiding a pumpkin seed (호박씨) that she had found on the ground. She says that she was so hungry that she had tried to eat it but found it to be nothing but an empty shell (쭉정이). After her husband hears her story, he is speechless. He walks over to his wife, hugs her, and starts crying, too.
The story may be the origin of 뒷구멍으로 호박씨 깐다, but the modern meaning of the expression would not describe the wife in this sad story. The wife was not really deceitful, just hungry and ashamed.
The literal meaing of 뒷구멍으로 호박씨 깐다 is "hull a pumpkin seed with the back hole." I always thought that "back hole" was referring to the "rectum," and maybe it is, but when I finally looked up 뒷구멍, I found that it also means "by unjust (unlawful) means." For example, 뒷구멍으로 돈을 주다 means "bribe a person with money." I will let you choose your own translation of the expression.
Also, in the Korean version of the story you will find the expression 빈 쭉정이, which means "an empty head of grain." Actually, 빈 means "empty" and 쭉정이 means "empty head of grain." Notice that "empty" appears twice, which means 빈 is redundant since 쭉정이 already means "an empty head of grain."
In case anyone is interested, I had never heard of the word, 쭉정이, until I read the above story.