Something that is extremely shocking or upsetting, particularly a newspaper headline or article. Also: marmalade dropper.
"BBC face obscenity row over shocking new lesbian drama," snorted the Daily Mail the other day. In Mail-speak, this kind of headline is known as a "marmalade dropper," meaning it's perfectly capable of bringing breakfast in the Home Counties to a spluttering standstill.
Aidan Smith, "Acting her age," The Scotsman, September 30, 2002
Fullalove by Gordon Burn, Secker Pounds 14.99 pp232. ...
You could argue, too, that the novel sometimes seems to be enjoying the horrors a little too much. But Burn has written a memorable, upsetting and clever book which is indeed to borrow Norman Miller's ultimate praise-phrase "a real marmalade dropper'," although it might be sensible not to read Fullalove over breakfast.
Mark Lawson, "Sleazy does it" (book review), The Sunday Times of London, August 27, 1995
This phrase has appeared almost exclusively in British newspapers and magazines, which makes some sense since marmalade consumption is common in Britain and isn't so common elsewhere (particularly in North America). The obvious New World equivalents jam dropper and jelly dropper yield no results. Some phrases that have appeared on this side of the pond (although not with anything approaching regularity) are muffin-choker, cornflake-choker, and coffee-spitter.