J-man means: A casual, irreverent nickname for Jesus Christ, used in informal or humorous contexts by people who want to reference the religious figure without full formality. It's the kind of term that turns up. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.