@pinkeree I think its from ' It's Fun Having a 300,000 Yen a Month Job Welcoming Home an Onee-san Who Doesn't Find Meaning in a Job That Pays Her 500,000 Yen a Month'
@nyanpass rakan's father ordered him to go somewhere supposedly for three months, which was extended to 3 years, when he returned she (Mao Mao's mother) had already (probably) caught the disease. Since she was no longer a virgin, her cost had been lowered considerably, so I think 'yubikiri' was...