ill
awesome
A colloquial youth-language expression borrowed from English that means 'extreme', 'impressive', or 'very good'.
Dein neues Skateboard-Video ist echt ill geworden!
Your new skateboard video turned out really awesome!

awesome / ill
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awesome
A colloquial youth-language expression borrowed from English that means 'extreme', 'impressive', or 'very good'.
Dein neues Skateboard-Video ist echt ill geworden!
Your new skateboard video turned out really awesome!

This word has two very different meanings, depending on the context. In youth language, 'ill' means something positive like 'super' or 'impressive'. In old literature or very elevated language, it means 'sick' or 'unwell'.
Der neue Song von ihm ist echt ill!
His new song is really awesome!
The youth-language meaning of 'ill' comes from English, where 'ill' can also mean 'impressive'. Do not confuse it with the main meaning of the English word 'ill', which is 'sick'. In German, the word 'krank' is used for 'sick'.
Deine Tanz-Performance war total ill!
Your dance performance was totally awesome!
In its youth-language meaning, 'ill' is almost always used predicatively, that is, after the noun with a verb like 'sein' (to be) or 'werden' (to become). Using it directly before a noun attributively is very rare.
Die Party gestern war richtig ill.
The party yesterday was really awesome.
The meaning 'sick' is outdated and belongs to elevated written language. Do not use this word in everyday language to say that someone is sick. It would sound very unnatural and old-fashioned.
In dem alten Brief stand, er fühle sich seit Tagen ill.
The old letter said that he had been feeling ill for days.
cool
'cool' and 'ill' can both mean something positive in youth slang: something is great, impressive, or very good. 'cool' is much better known in German and is understood in many casual situations. 'ill' comes more strongly from English and sounds very youthful, specific, or scene-based in German. Learners may confuse the words because both can have a positive meaning. As a practical rule: 'cool' almost always works in casual language, while 'ill' only works when a very modern or English-influenced youth style is intended.

Der Trick auf dem Skateboard war echt ill.
The skateboard trick was really awesome.
Im Technikladen fand sie den kleinen Lautsprecher richtig cool.
In the electronics store, she thought the small speaker was really cool.