#BBCtrending: Why is #oomf so popular on social media?
There is an enigmatic hashtag - #oomf - that repeatedly trends on social network sites, and in particular Twitter.
It has been more than 150 million times since it first came into use, in the spring of 2010.
But what does it mean? And when do people use it? #BBCtrending investigates.
Archive courtesy British Pathe
Produced by Dougal Shaw
All our stories are at BBC.com/trending