Arguably means as a case could reasonably be made. Arguably the best, arguably true, arguably a mistake.
It is a hedge that lets a writer make a strong claim while conceding it is contestable, which is why critics and sportswriters lean on it. Saying something is arguably the best invites the argument rather than closing it.
1adverbarguablyAs a case could reasonably be made.
“The bicycle is arguably the most useful invention of the last two centuries.”
arguably is a manner.
From arguable + -ly.
Etymology from Wiktionary, CC BY-SA