In a manner that shows unfair preference or partiality; in a biased way.
From the adjective 'biassed' (British spelling) plus the adverbial suffix '-ly'. This formal adverbial form allows modification of verbs to show bias in action.
This is a rare but perfectly grammatical adverb—you could write 'the referee biassedly made his calls,' showing that bias can be an action, not just a state of mind!
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