IPA: /kiːp ˈ keɪvi/, alternately 'keep cave' /kiːp ˈ keɪv/
Often abbreviated 'KV,' the phrase 'keep cavey' is British school slang thought to originate at Eton College in the 19th Century. It is a corrupted pronunciation of the Latin language command Cavē! meaning 'Beware!' or 'Stay vigilant!' (IPA: /kaˈ veː / or /kaˈ weː /).
"Keep cavey; the teacher's headed this way!"
"KV if you're gonna' smoke behind the woodshed."
"Brian, mind keeping cave for me while I hide my stash?"
Migration and Evolution
The abbreviated form 'KV' - usually explained, when asked, as standing for 'Keep Vigilant' - has found its way into the modern slang of traveling carnival workers and amusement park staff in the United States. Its use in these contexts varies from looking out for pickpockets and shoplifters, to watching for small children who might be separated from their parents. It is also used as a 'heads up' between ride operators, to double-check each passenger has secured their safety belt, and to alert carnival security officers to belligerent guests.
Over time, the use of KV in this context has developed a second meaning, 'Go make yourself useful somewhere else, out of my way,' and a third meaning, 'I am about to step away for an indefinite span of time, so you should man my post.' Due to the practical significance of the phrase in its primary meaning, the speaker has plausible deniability against accusations of using the phrase rudely, which makes KV one of the more passive-aggressive ways one carny can impose on another's time or tell them to go away.
An equivalent phrase in use by the general public, without the passive-aggressive additional meanings, is 'If you see something, say something,' which can be found on public safety and awareness posters throughout US cities.
Iron Noder 2019, 1/30
Wertperch's Perpetual Noding Challenge