The most obvious meaning as「sex slave, cunt, female crotch efforts」refers to the daily life of a sex slave TOILING in ancient China, but also to pimping a child sex slave most anywhere today. The English usage of 'hard' to mean 'difficult' originated with some material being hard and therefore difficult to penetrate, likely stuff located within a compacted, constipated anus associated with a female crotch, or a body in a state of rigor mortis…or both for that matter.
　「Female crotch efforts」best describe AS MUCH EFFORT AS POSSIBLE, TOIL, STRIVING, DILIGENCE, ENDEAVORING…maybe in reference to males bar-hopping or hopping on sex slaves (「power with cunts」), but also to females during the process of giving birth. Childbirth was the leading cause of death for young females, who began birthing at about the same time as the cycle of menstruation and ovulation started in the early teenage years while the mother's skeleton and orifices were still small and subject to severe tearing.
　Perhaps, a reference was to get the MOST EFFORT POSSIBLE out of one's「female slaves, prostitutes」by working them to the point of death, prostitution being a form of slavery especially when controlled by a pimp. Squeezing the last possible ounce of energy out of their TOIL, STRIVING, DILIGENT EFFORT, ENDEAVOR, not unlike the business world of many employer-employee relations today where 'this job sucks.'
　After any battle, the enemy older males were likely slaughtered and decapitated, but the females and young boys were taken as various sorts of slaves.
　The concept of doing something physically and never becoming tired or giving up also applies to「female crotch effort」since the human female is physically able to copulate continuously with many multiple partners in a seemingly endless succession, with no respite between partners (…passing out is a distinct possibility, however).

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 exert; to strive
Japanese: never, by no means, to strain, to bear up, to exert one's strength, to swagger, to bluff, to boast, certainly, absolutely, a little bit, diligently, as much as possible, make an effort!, work hard!, to endeavor, to try, to strive, to make an effort, to exert oneself, to be diligent
Unihan extended: to exert, strive, make an effort; to pout
EDRDG: toil; diligent; as much as possible