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By Allah (Arabic: Wallah, والله) is an Arabic expression meaning "[I promise] by God" used to make a promise or express great credibility on an expression. It is considered a sin among Muslims to use this phrase and follow it up with a lie. Also, some Muslims argue that this is one of the few valid ways of making a promise, the variant "I promise on myself" or "I promise on my own soul" not being allowed, in contrast to "I promise by the one who holds my soul", i.e. God. An alternative is Wallaahi, "By my God", often contracted by non-Arab populations in Africa to Wallai.
Another meaning is "Really?", "Is that so?" (referring to "Do you swear on it?", used in spoken Arabic), "By God!" (this meaning has been adopted by Modern Hebrewslang as well).