[mrebus]

Words flipping from positive to negative and visa versa is very common in language. Just the other day I was talking to some younger kids about hockey and they were going on about a goal being "filthy" and "Dirty". In my day a dirty goal was one that didn't look nice. To them these goals were so naughty(as in sexy) you wanted to take a shower after seeing them.

Here's some reading http://www.dailykos.com/story/2014/03/15/1284995/-Origins-of...

[sk5t]

I don't think this particular case is a new-ish usage; "filthy" has long been popular in baseball to describe a very crafty pitch or pitcher, or "sick" in music for an amazing solo or musician.