It means someone you played against previously got caught cheating (by using an engine, for example) and their account was banned. Doesn't necessarily mean they cheated in their game against you - could have been any game - but you get awarded some rating points just in case.
Fair Play Policy
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Can anyone tell me what this means? How can an opponent violate the FP policy? I have received three of these messages now. What are my opponents doing??
'We have detected that one or more of your recent opponents has violated our Fair Play Policy. As compensation for potentially unfair rating losses, we adjusted your following ratings:'