Jack the lad means: An irresponsible young man, seeking personal pleasure and gratification without any regard to responsibilities; a rogue.. It is informal slang, fine in casual speech but not suited to formal contexts. 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.