Admiral of the fleet means: The highest rank in the Royal Navy and other Commonwealth navies, NATO grade OF-10, currently only ceremonial.. It is military jargon and mostly appears in service contexts, war fiction, or among veterans and their families. 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.