“im Knast” is rather colloquial right? A bit like saying “in the can” or “in the slammer” in American English?
English Translation
In the end, he landed in jail.
Yes, “Knast” is informal. The dict.cc entry lists a bunch of mildly comedic synonyms, including some I haven’t heard in years, like “hoosegow”, “pokey”, “big house”, and also the two you mentioned, “the can” and “slammer”: dict.cc | knast | English Dictionary