The Spanish word ojalá, meaning I hope so, comes from the Arabic for God willing.
It descends from law sha Allah, and is one of roughly 4,000 Spanish words with Arabic roots. Others include almohada, aceite, azúcar, alcalde and barrio.
Why is this true?
Muslim rule in parts of the Iberian Peninsula lasted nearly 800 years, from 711 to 1492. Many of the borrowed words begin with al, the Arabic definite article that came along with the noun.
Source: Wikipedia: Arabic language influence on the Spanish language
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Language facts show how much of everyday speech is historical accident. Words that once meant the opposite, letters that vanished from the alphabet, punctuation with names almost nobody uses. Each entry explains the change that produced the oddity — a sound shift, a printing convention, a borrowing from another language — so it works as a genuine explanation and not just as a piece of trivia.