From Pokemon Schwarz:
Dein Pokemon-Team ist wieder topfit!
I can't find any reference to 'topfit' so I'm guessing 'top-' is just a prefix meaning 'sehr' or maybe something like the English 'tip-top'. Is this a colloquial German prefix that can be added to words, does it occur only in 'topfit', or is this a word they just made up in the game?
Yes, this one way to express the superlative. Duden calls it umgangssprachlich emotional verstärkend. It doesn't work with all adjectives, but there are a handful: topgesetzt, topgepflegt, topqualifiziert. Again, Duden has more.
It can be used with nouns as well:
Der Teilnehmer erzielte eine Top-Platzierung in der Wertung. Das Büro befindet sich in einer Top-Innenstadtlage.
I think it originated from English and is used sparingly in German. From what I can tell by looking at beolingus and wiktionary it is a word originating from English, used as an adjective in German. Therefore you should be able to prepend it to German words, but since it's originally English I would assume it not best practice to do so liberally.
Literally "top-" means Top in German, and in this context "topfit" simply refers to "top-shape" or better translated to "well rested/healed" in the Pokemon world.
In my opinion, "top" is first and foremost an adverb.
top gepflegt, top erhalten
As @Ingmar mentions in his answer, the Duden lists these words as one word but I think this is... nonsensical, Duden or not. Duden is just A dictionary after all.
In some (very rare) occasions the "top" can be fused with the word but "topfit" is the only one I can think of right now, which by the way would be written as two words in English.
"Top" also shows characteristics of an non-declinable adjective.
Das Auto ist in einem top Zustand.
I see no reason why to write that with a hyphen. First of "top" is clearly answering the question "what kind?", which is what adjectives do, but even more important: a sentence like the following (a blue print predicate adjective) is perfectly valid.
Das Bier war top.
I think the hard to pin down nature is indeed due to its English origins.
So... is it a prefix? In my opinion ... no. Can it be used freely? Again, no. It is more flexible as an adjective but as an adverb (superlative-maker) it is really rare. Hard to say when it works and when it doesn't but if it doesn't it will sound really out of place.
Das Auto ist top schnell... what?
