"Felony" like. Up to no good.
Any activity you could probably catch a felony by participating in.
1."What's that guy doing behind that building?"
"I don't know, but it looks pretty FELONIOUS."
2."She's the biggest troublemaker I know, totally FELONIOUS."
Since felonies are considered crazy( or calamari )
felonious is a slang term for crazy
That bitch was felonious!