A「necessary flow」of some sort, referring to fluids that are SECRETED from urinary organs (we all must do that at least once daily, it seems), or to mucus that OOZES slowly from bulbourethral glandular bodily orifices (such as, Cowper's glands or Bartholin's glands), a「flow necessary」and vital for coitus and successful semen transmission. A「flow necessary」also describes that of semen for most any male of any animal species, and the elements as「inevitable flow」may also directly reference that of semen, since if there is no alternative relief, nocturnal emissions will be experienced as a natural consequence of overloading, filling beyond rated capacity.
　Actually though, while admittedly unpleasant to contemplate, the most likely meaning is gleet「flowing from a penis infected, invariable」with syphilis, chlamydia, and/or gonorrhea as a STEADY FLOW, a SECRETION that OOZES a constant purulent discharge.
　NASAL MUCUS was well known as prime starting material among females desiring to masturbate. A fact that perhaps only zookeepers are aware of today, the nose is generally a fine place to readily locate a source of mucus needed for lubrication when bonobos or other animals, such as human beings in the past, start to perform some「self-conferring of the prize, getting some self-booty」(鼻)…both male and female bonobos have learned to blow snot into a hand and use it to begin masturbation, and this technique is deliberately taught to the young. See 齆 and 鼽
　Without a doubt, the Irish also have knowledge of their own to pass along in these trying times:「Masturbation will not spread coronavirus, especially if you wash your hands (and any sex toys) with soap and water for at least 20 seconds before and after.」Nonetheless and regrettably, unlike in dynastic China, it must be undertaken alone to truly be a preventative countermeasure but still achieve the desired result with some degree of safety.
　 　[　https://www.sexualwellbeing.ie/sexual-health/sex-and-coronavirus/ 2020-03-31　]　
　The Dutch, too have the same solution—it seems masturbation has become mainstream in the media the world over due to the coronavirus (it has always been 'the main stream' in reality, albeit covertly for most folks):
　「'Don't have sex with your partner if they have been isolated because of (suspected) coronavirus infection,' the RVIM says. 'Sex with yourself or with others at a distance is possible (think of telling erotic stories, masturbating together).'」
　　[　https://www.theguardian.com/world/2020/may/15/dutch-official-advice-to-single-people-find-a-sex-buddy-for-lockdown-coronavirus 2020-05-16　]　
　Needless to point out perhaps, but Chinese shamans from antiquity hit upon the same solution as a pandemic preventative countermeasure, honed for ultimate client satisfaction by utilizing naked females with no-longer-integrated feet as masturbatory sex buddies available for short-term rental with erotic visualization of live prepubescent female genitalia engaging in autoerotic stimulation herself, as a solution to the same problem facing us now, but in pre-Internet days.

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: to secrete; to excrete
Japanese: to pierce, to penetrate, to soak in, to permeate, to be infected, to be steeped
Unihan extended: to seep out, excrete
EDRDG: ooze; flow; soak in; penetrate; secrete