What it means
The British twin of loose lips sink ships, another wartime poster slogan warning that a stray word in a pub or on a bus might reach the enemy. It survives as a tongue-in-cheek nudge to keep sensitive news to yourself before it spreads.
Usage examples
"Careful what you say at the bar about the redundancies, careless talk costs lives and half the office drinks in here."
"Don't post the holiday dates online, careless talk costs lives."
"He clammed up about the merger, muttering that careless talk costs lives."
"I know you've got the tea on who's leaving, but careless talk costs lives, so zip it till HR says something."
"Mum heard me mention the surprise party in the kitchen and just went, careless talk costs lives, you little gobshite."
Where it comes from
This one comes straight from a British Second World War propaganda campaign. It was printed on posters to warn civilians not to chat loosely in public, since a casual remark could be overheard and passed on to the enemy. After the war, it stuck around as a dry, knowing way to say keep it quiet.
