Without any doubt or in a way that cannot be argued against; absolutely certainly.
From 'un-' (not) plus 'deniably' (from Latin 'denegare' meaning to deny). The prefix and root combine in Middle English to create an adverb meaning something so true it's impossible to deny.
This word is stronger than 'definitely' because it puts the burden on other people—if something is undeniably true, YOU can't deny it, which makes it philosophically sneakier than just saying 'it's definitely true.'
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