Definition of arguably adverb from the Oxford Advanced Learner's Dictionary

 

arguably

 adverb
adverb
BrE BrE//ˈɑːɡjuəbli//
 
; NAmE NAmE//ˈɑːrɡjuəbli//
 
 
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used (often before a comparative or superlative adjective) when you are stating an opinion that you believe you could give reasons to support He is arguably the best actor of his generation.
See the Oxford Advanced American Dictionary entry: arguably