Codi pais ar ôl piso

Literally: Lifting the petticoat after peeing

Example: “Does dim pwynt codi pais ar ôl piso; mae'r trên wedi mynd.” — There's no use crying over spilled milk; the train has gone.

English equivalent: Cry over spilled milk

Origin: Lifting one's petticoat after having already peed is taking a precaution too late. The phrase is a blunt, humorous Welsh saying for acting when it no longer helps, like English "shutting the stable door after the horse has bolted."