The archetypal Radge is wild, unpredictable, and violent. A man you do not want to spill a pint on.
As well as being Radge, you can also go Radge: a prolific, sustained bout of swearing or a violent outburst ought to do it.
It’s also used as a casual term of endearment, predominantly in the east coast (“come here, ya wee radge.”). A seemingly improbable event can also be described as being “radge”.
A word that dates back to the late 19th century, radge is probably a morphing of “rage”. It was adapted from radger, a seemingly extinct term for an uncontrollable creature.