The Spanish phrase "dietas perder peso rapido" roughly translates as "diets to lose rapid weight" in English, according to Online-Translator.com. "Rapido" means "fast" in English, so the phrase is more clearly translated as "diets that cause a person to lose weight fast."
One of the ways Dictionary.com defines the word "diet" is as a selection of food designed to improve a person's health or physical condition. Both the Spanish and English words for "diet" come the Medieval Latin word "dieta," which refers to a "daily food allowance," among other things. The word is also rooted in the Latin word "diata," which means "prescribed way of life."