There is a story in China, where King Huai of Chu (懐王) saw a dream where he laid with a goddess from the mysterious Wu Mountains (巫山). Now, the character 戯 (as well as ふざける!) can also mean "to have intercourse". Combining "Wu Mountains" and "intercourse" together gives us a story within a single word - 巫山戯る.
As for the reading: the first part, 巫山, is read ふざん. It would be difficult to say ふざんける, so the ん was dropped. The ける part, however, was made up for newer Japanese (the word before was ふざく and didn't have space to house the 戯 character). It could've been ふざかる or something else, for all we know. The point is 戯 doesn't have a reading け・る outside of the word ふざける. And that's pretty curious.

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