Was this a serious question? Whilst not wishing to introduce the seamier side of life into proceedings, the expression has direct sexual connotations I'm afraid. All I'm prepared to say is that it refers to a rather primitive form of contraception.
Have to disagree with Joko and agree with Blue Peter...in my younger days the saying was "one up the bum, no harm done" - as BP says, the "no harm" meaning that the woman wouldn't get pregnant...