When it comes to shady individuals or businesses that cannot be trusted, "fly-by-nights" is an old-fashioned term that still has some relevance. However, there are many other words and phrases that can be used as synonyms for this term. For example, "hustlers," "scammers," "con artists," and "charlatans" are all similar in meaning. Other options include "grifters," "swindlers," and "shysters." There are also more specific terms that can be used, such as "loan sharks," "ponzi schemers," and "pyramid schemers." Regardless of the term used, the message is clear: beware of those who operate under the radar and cannot be trusted.