born on the wrong side of the blanket

used to refer to a child whose parents were not married at the time of birth

The phrase "born on the wrong side of the blanket" is an older idiom that dates back to the 18th or 19th century. It alludes to the idea of a child being born out of wedlock or to parents who were not married at the time of their birth. This phrase is used to describe someone's birth status, emphasizing their parentage outside the bounds of traditional marriage. The phrase has fallen out of use in recent years, as attitudes towards illegitimacy have become more accepting, and the term itself is now considered outdated and potentially offensive.