"mother's lock" 婆婆上锁

"the most important source of social support for pre- marital sexual intercourse comes from the parents of young men. Premarital sex not only reduces the financial burden of wedding costs, but it also virtually insures the marriage. Thus, parents of young men often encourage their sons to engage in premarital in- tercourse. In Wuhan, the new term "mother's lock" (popo shang suo) best illustrates this phenomenon. This refers to the practice whereby, in order to encourage her son to have sex with his girl- friend, a mother locks the young couple inside the bedroom"

"Virginity and premarital sex in contemporary China", Xiao Zhou, 1989.