Leviticus 15:2

Speak with the sons of Israel, and say to them: A man, whether young or old, who hath a defluxion from his flesh, when he hath seen it three times, is unclean.

flow as in, “flowing with milk” [Exodus 3:8] — i.e., trickling drops

from his flesh a euphemism for the male genitalia

his flow here, a noun

זב מבשרו, our sages understand this as looking like a mixture of seminal fluid and a discharge which is the result of that organ being diseased.

זב מבשרו, the Talmud K’ritut 8 already explained the preposition מ in the word מבשרו as meaning “emanating from,” and not as “due to, on account of.” The word בשר refers to the male organ. In other words, unless the symptoms described here are due to a local physiological imbalance and not as a secondary effect of some other primary cause such as overeating, for instance, these symptoms will not result in and confer ritual impurity.
Our sages in Niddah 35 already explained that the symptom called זוב is very similar in appearance to the albumen in an egg which has become rotten. When the cause is directly connected to the male organs of the afflicted person the legislation under discussion in this chapter applies. This is the meaning of the word מבשרו, a disturbance in the male organ to function properly. The author speculates that the cause may be overly frequent indulgence in the sex act, which in turns leads to insufficient time for the seminal fluid being replaced properly, etc. If that were the cause, a cause brought about by fantasizing about the sex act too often, the remedy, in part, would be the seven days during which the afflicted person counts towards his being healed, reflecting on the part his lifestyle played in causing the symptoms from which he suffered. By indulging in fewer sexual fantasies and the need to offer a sin offering after being free from these symptoms the afflicted person my be on the way to spiritual rehabilitation also. These thoughts are reflected in the Talmud on the folios we referred to.