The colloquial verb for sending someone to prison is verknacken, that's why (Ex-)Knacki means "jailbird".
So is Knäckebrot "prison bread"? It would be quite fitting.
Er... no. "knäcka" is Swedish for "to crack" (I think).
Since there is no vowel change in (ver)knacken, those two words don't even look similar to us. (Just like no Spanish speaker will understand how anyone could ever confuse "velo" and "vela" or "pollo" and "polla" - they're just totally different words to them.)
See I didn't suspect Swedish because I'm learning it and bread is bröd. But now I looked it up and it's partial calque of Swedish knäckebröd, half of the word was changed to "brot". That's why I assumed it was German so thought there must have been a vowel change from knack.