As a non-native speaker, I already get used to the word enough in expressions like those below, but I sometimes still got confused of it. It makes me wonder what it actually means and where does it come from.
There are two different groups in which it confuses me. Actually, the first group contains only one expression, that is fair enough. This is an idiomatic expression that is 'used to admit that something is reasonable or acceptable' Oxford Dictionaries.
The second group contains hundreds of examples; actually, it's a seemingly endless list:
funny enough, oddly enough, strangely enough, interestingly enough, curiously enough, amazingly enough, appropriately enough, astonishingly enough etc.
Frankly speaking, in my mother-tongue, German, there's something similar, i.e. we add -weise to certain words:
interestingly: interessant -- interestingly enough: interessanterweise
amazingly: erstaunlich -- amazingly enough: erstaunlicherweise
Though, there are two main difference between enough in English an -weise in German. First, -weise does not have any meaning on its own1 and, second, in those instances you use -weise you cannot drop it whereas in English you don't necessarily need to add enough. Example:
English: Interestingly (enough), you don't need *enough* in this sentence.
German: Interessanterweise kann man in diesem Satz *-weise* nicht auslassen.
(Interestingly, you cannot drop *-weise* in this sentence.)
In all these expressions, the general sense of enough (sufficient, to the necessary or required degree) cannot be applied; at least, it wouldn't make any sense in my book.
Funny enough, for example, means that a fact is funny but not that there's a sufficient amount of fun.
So, again my question: what's the role of enough and why that word?
1For the sake of completeness I must mention that this statement is not really true. There's an adjective weise meaning wise (a wise old man) and a noun Weise meaning way as in a certain way or a mysterious way. When talking about the suffix -weise in adverbs, the latter meaning (the noun) can be taken as an indicator what -weise may mean; admittedly, I've never validated this from an etymological point of view.
Best Answer
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.