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The element for-, though it stems from the same Proto-Germanic word that gave us the preposition for, deviated from the common ancestor to serve as a prefix meaning “away,” “opposite,” or “completely.” That’s the sense that contributes to the meaning of most words beginning with for-. Notice that these words have in common that their connotations are definitive: The verbs forbid (“prohibit,” with a root cognate with bid and meaning “command”), forget (“fail to remember” or “inadvertently neglect,” with a root cognate with get and meaning “grasp,”), and forgive (“pardon,” with a self-evident root) are potent; so, too, is the adverb forever (“always,” with a self-evident root).
Source: What “For-” Is For