Old element of「using two feet in a stance」: elements today seem to depict a merging of glyph markers with「tense, marker in a no-longer-integrated foot」or「clever」(刁) as the upper-left element and「spurting」as the upper right. Meaning「two feet」in a STANCE not walking away, often in an unstable position due to having the focus of one's concentration directed elsewhere, perhaps while performing spasmodic thrusting motions with one's pelvis. Unstable as were the no-longer-integrated feet of a foot-bound concubine herself, wrapped tightly in long cloths. Inner element formerly was not 'insertion, put into' (入) although in some fonts it appears that way today as「two, insertion or put into, one.」Scholars explain that the left and right sides were「meat feet」(from 舛).
The analogy between women kept cooped up in a harem to that of chicks and birds with one cock continues:「The hen will crouch (dip her head and body) to indicate receptiveness to the male [lordosis position]. The rooster will then mount the hen and grab her comb, neck feathers, or the skin on the back of her head or neck to help hold onto the hen's back. The next behaviour is the tread [he does his gigue ('gig'), or dances a jig] (the rooster walks quickly in place on the hen's back) and finally the completed mating of the behavioural sequence [inverted everted meat of the 'cloacal kiss']. …The rooster usually ejaculates between 100 million and five billion sperm at a time with greater concentrations produced at the beginning than at the end of the day, when depletion occurs after many matings. …The frequency of mating follows a diurnal pattern with mating frequency reaching peaks early and late in the day [evening meat with feet]. A rooster may mate from 10 to 30 or more times per day, depending on the availability of hens and competition from other roosters. However, the number of sperm per ejaculate is seldom less than 100 million which is the minimum required to maintain high fertility. …The rooster has a small phallus that becomes engorged with lymph to form a copulatory organ. The copulatory organ is rudimentary and at the time of mating there is practically no penetration.」
[ https://www.thepoultrysite.com/articles/1330/natural-mating-and-breeding 2014-09-29 ]

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Unihan extended: legs