Fixin' to means: About to; preparing to. "Fixin' to leave" = about to leave. Quintessentially Southern, particularly Texan and Appalachian.. 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.