Why does 皮肉 mean "irony"?

Question

I gather that 皮肉 can literally mean "skin-meat." I also see that one definition for 皮 is "mask (hiding one's true nature);  seeming." So perhaps 皮肉 can be understood as "hiding the real meat," which can be related to irony. What is the real answer?

Answer 1

snailplane's link says, it comes from 皮肉骨髄 "skin meat bones marrow" attributed to the Bodhidharma of Chinese Zen Buddhism. Bones and marrow came to take on the meaning of essential, skin and meat became synonymous with superficial. From there, 皮肉 was also used as a word for criticizing faults/defects (which stems from not recognizing the true nature of sth.), which seems to be its primary current usage, e.g. the JDIC dictionary entry for the derived [皮肉]{ひにく}る says:

皮肉る
to speak cynically or with sarcasm

Although 皮肉 is sometimes best translated to irony, as in the fixed expression 皮肉なことに, e.g.

バイトを辞めたんだけど、皮肉なことに、年中他の仕事でずっと忙しかった。
I quit my part-time job, but ironically was busy with other work all year.

I think 皮肉 works best for cynicism/sarcasm or dark/pessimistic irony, rather than the lighthearted variety of irony.

Answer 2

皮{ひ}肉{にく} literally means "skin and flesh" and comes from the idiom 皮{ひ}肉{にく}骨{こつ}髄{ずい} ("skin, flesh, bones, and marrow"). This word comes from Chán Buddhism in ancient China, and refers to a legend about how the dharma passed from Bodhidharma to Huìkě.

As the story goes, Bodhidharma wanted to return to India, so he had to choose a successor. To do this, he asked each of his four disciples to express their understanding of Chán. The first three expressed their understanding in words, but the last, Huìkě, simply bowed in silence. Bodhidharma's judgment was that the first three had attained his skin, flesh, and bones, but Huìkě had attained his marrow, and so he passed the dharma on to Huìkě.

Although this legend has multiple interpretations, only one is relevant to your question: the four body parts represented a hierarchy from most superficial understanding (skin) to the most essential (marrow). From this, 皮肉 came to symbolize superficiality in general, while 骨髄 came to mean "essential", "core". (Note that while 骨髄 means "bone marrow", it still has the figurative meaning as well.)

For the rest of the story, please see Earthliŋ's answer.