SBE
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In my example, the last time I heard it I was about ten and I had just had an accident that required a rush to A&E for lots of stitches and I was crying and the doctor told me that as a joke. So for an injury that might leave a mark. Implying that by the time the child becomes old enough to look for a marriage partner (pc all the way), the injury will have healed and there would be no disfigurement that might repel potential suitors.