¨Where people put an edge on their crowbars¨ Well, it is not a common saying so I googled it as see that it comes from a Terry Pratchett book. Whenever you come across saying you don´t know try and look at the context to see if you can figure out what they mean In this case, he is in a place there are a lot of thieves. A crowbar would often be a tool carried or used by thieves, to open things etc. You may expect to find them in this type of place Now when you sharpen the end of a crowbar you get a knife-like-edge, so the crowbar now becomes an even more dangerous weapon. So we can conclude that if you are in a place where people put an edge on their crowbars, you are in a very dangerous den of thieves where the thieves will not only steal but murder, or at least injure, if needed and will do it without a second thought.

I'm from the USA and I've never heard this phrase. However, if I heard someone say it, I would think they were describing a dangerous neighborhood. Crowbars are used to break into homes, so if someone used one with an edge, they might be breaking into a home with a violent intention.

I have never heard this phrase before. 'Edge' most likely refers to the edge of a blade (the sharpened part). So the writer is saying that the neighbourhood is so dangerous that people sharpen their blunt weapons.