Which is the proper spelling: "disfunction" or "dysfunction"?

Dysfunction is by far the more common according to Google Ngrams

Hugo found a useful thread at Word Reference.

It points out that dys- is Greek in origin, and means "bad or difficult" and that function is Latin in origin and means "an activity that is natural to or the purpose of a person or thing". It goes on to say that the combination of Greek and Latin is somewhat odd, especially since dys-'s homophone dis- is Latin (but it does not mean the same as dys-).

The dis- prefix doesn't work for dysfunction because its meanings (as explained here) are not the same as dys-, so they would change the meaning of dysfunction.

The correct spelling is dysfunction. It would not surprise me, however, if disfunction became an accepted spelling, because it just seems more normal.

If you try searching disfunction on the OALD, you can see it won't find it.

Same goes for the NOAD. The only one that had it signalled as variant is the Oxford English Dictionary.

Now, this doesn't necessarily mean that the word doesn't exist, but I'd interpret it as a signal that the NOAD and the OALD consider it to be non standard or at least not common enough.

So, if you say dysfunction, you're safe.