ㅋㅋㅋ I remember well this abbreviation from hair to head.
머리 잘라 주세요 = please could you cut my head <-> hair ...
Since I asked that question, I have learnt that there are a few ways of describing a "Bad hair day".
일이 잘 안 풀리는 날 = Day that does not go well / that things don't work out well
그저 그런 날 = one of those days
재수없는 날 = unlucky day
Pity, none of the expressions has 머리 in it ...