Gadfly means: Any winged]] insect or fly of the family or (horsefly), noted for irritating animals by buzzing about them, and biting them to suck their blood; a gadbee.. 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.