Able to be described, proved, or supported with detailed and specific evidence or circumstances.
From circumstantiate + -able suffix. The root circumstantiate comes from circumstantial, which derives from circumstance (Latin circum- + stare). The -able suffix means 'capable of being.'
A claim is circumstantiable when you can actually back it up with the surrounding details and facts. This word is the legal world's way of asking: 'Can you substantiate this with evidence, or is it just speculation?' It's what separates gossip from testimony.
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