A quick Google suggests its a North American thing.... the term seems to be "granny gas".
There seem to be multiple mentions of it on one Canadian website in particular (stick
site:avcanada.ca Granny Gas into Googoo)
One post on that forum even goes as far as stating the following......
Unfortunately the concept of “granny gas” has become an industry standard, particularly for operations north of the 60th parallel. After conduction many long range trips with “granny gas” on board, the pilot eventually will consider this normal operation and not consider this to be a foolish act.
Or another example from the many on that website.....
There seems to be some confusion over the term, "granny gas". Contingency fuel is that above the normal legal amount the crew decides to carry over and above the legal minimum. Granny gas, is that extra 45 minutes the crew "hides" from the prying eyes of the dispatcher. It remains incognito, so to speak until that rare day when it's needed. It used to be very, very, very common practice. I know it saved my bacon once. I'd almost forgotten the extra 60 gallons I "hid" in the Racer, till one day, the tanks we were using blew