In a way that is open to debate or discussion; questionably or arguably.
From debatable + -ly (adverbial suffix). Debatable comes from debate + -able (adjective-forming suffix), creating a chain of affixes showing how modular English word-building works.
This word is perfectly self-referential—saying something is 'debatably true' means it's literally debatable whether it's true, which is delightfully recursive!
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