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Related to rule in: Rule of 72, Rule of thirds, Rule britannia
To decide definitively that something will or can possibly happen. (Used especially—and somewhat redundantly—in the phrase "not ruled in or ruled out.") Look, nothing has been ruled in or ruled out at this stage, so we just have to wait until the doctors have more information to work with. We'll neither rule in nor rule out the possibility of a shakeup in management if this branch's profits continue to fall.
See also: rule