[Mithalwen]

There is the expression "do yourself a mischief" meaning injure or harm yourself.
Not all phrases are well documented. My mother used to call the little triangular patches of grass at a fork in a lane "god cakes". I tried to look it up and it took a while - I began to think it was a "motherism" but I then found there was a little triangular local pastry in her native Warwickshire called a god-cake ,,, but it took a bit of digging and even then the only refs so far are in a cookery blog and a Leaminton Parish magazine...