Both the Spanish reírse (“to laugh”) and the English ridiculous come from the same Latin root: ridere (also “to laugh”).

Thus, the r‑vowel-d-vowel of ridiculous maps to the r‑vowel-disappeared-vowel of reírse. Note that the middle ‑d- disappeared in the Spanish version, probably as the word was shortened since the Spaniards spent so much time laughing, it became natural to say it shorter and quicker!