Fausset's Bible Dictionary
Issue Out of the Flesh
i.e. from the generative organ; the result of undue indulgence of lust, enervating the organs (Leviticus 15:1-15; Leviticus 22:4; Numbers 5:2). For seven days after its cessation the man was unclean, and then must bathe, wash his clothes, and offer sacrifice. The severe form of the disease is modern, appearing first in the 15th century and more virulently in the West than the East.
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Fausset, Andrew R. Entry for 'Issue Out of the Flesh'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/i/issue-out-of-the-flesh.html. 1949.