Ponerse como una sopa

Literally: To get like a soup

Example: “Salí sin paraguas y me puse como una sopa.” — I went out without an umbrella and got soaked to the skin.

English equivalent: Soaked to the skin

Origin: A sopa was originally a slice of bread soaked in broth, and the dish took its name from it. Getting as wet as a sopa is being soaked right through, like that bread. Spanish also says estar hecho una sopa, and French has the same comparison in trempé comme une soupe.