What is the legal definition and origin of Zihar (involuntary divorce by analogy to the back)?

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Zihar is linguistically derived from the term 'back' (dhahr). The specific analogy to the back among all limbs is made because whatever is ridden upon is typically called a back, and thus the wife is likened to that. Zihar is forbidden (muharram). Its prohibition is established by the Quranic declaration: "And indeed, they are saying a reprehensible statement and a falsehood" (Quran 2:2). The meaning of Zihar is declaring the wife as unlawful to the husband as his mother is unlawful to him. This is negated by the Almighty's statement: "They are not their mothers" (Quran 58:2), and further clarified: "Allah has not made those whom you declare [as unlawful to yourselves] by saying 'You are to me as the back of my mother' the mothers of your men" (Quran 33:4). The primary basis for Zihar in Islamic law derives from both the Book and the Sunnah. Regarding the Book, it is established by the verse: "Those who declare from among you their women as unlawful to them, [by saying] 'You are to me as the back of my mother,'..." (Quran 58:2) and the subsequent verse.