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"Wang" is a slang term for a penis.
According to the Online Etymology Dictionary, "wang" probably originates from whangdoodle, an older term for "gadget, thing for which the correct name is not known." Penises have long been named similar words, like "thingy" or "dingus," so it's not unlikely that this word was applied to penises.
Another possibility is that it comes from the Renaissance English whang, meaning "large, thick slice" although the word "wang" itself as a slang term for penis appeared in the 1930s.