What does "enough" mean in expressions like "Fair enough" or "Funny enough"?

In my opinion, the adverb enough used in this sense creates a conversational rapport between the speaker/writer and listener/reader. In strangely enough, interestingly enough etc., the enough might mean "enough to go so far as to consider it such". The reason it's used is perhaps a fear that the addressee might not feel the described circumstances really warrant the qualification strangely/interestingly/etc. By using it, the speaker/writer acknowledges that their listener/reader is right to think that, but that the circumstances really can be considered such for the sake of their conversation.

In any case, this enough is not used in a formal register. When it is used in a formal setting, it signals a (temporary) lowering of the register.

Just my two cents.