Relating to, involving, or characteristic of extortion; used for purposes of extortion.
From 'extortion' plus '-ary' (suffix meaning 'relating to'). Similar pattern to 'dictionary' (relating to diction) or 'military' (relating to militia).
English has multiple suffixes for 'relating to something'—you could say 'extortionary,' 'extortionate,' or 'extorsive,' which is why we end up with so many similar words.
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