something nasty in the woodshed

something nasty in the woodshed

a shocking or distasteful thing kept secret. British informal
This expression is taken from Stella Gibbons 's comic novel Cold Comfort Farm ( 1933 ), in which Aunt Ada Doom's dominance over her family is maintained by constant references to her having seen something nasty in the woodshed in her youth. The details of the experience are never explained.