“He was smart if not exactly brilliant.” In this sort of expression, “if not” links a weaker with a stronger word with a related meaning. Other examples: “unattractive if not downright ugly,” “reasonably priced if not exactly cheap,” “interested if not actually excited.”
But this sort of “if not” is often misused to link words that don’t form a weaker/stronger pair: “obscure if not boring,” “happy if not entertained,” “anxious if not afraid.” The linked terms in these examples do have some logical relationship, but they do not form a weaker/stronger pair.
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