their

Etymology

From Old Norse þeirra. Replaced native Old English heora.

det

1. Belonging to, from, of, or relating to, them (plural).
   they will meet tomorrow at their convenience
   this is probably their cat
   For Liverpool, their season will now be regarded as a relative disappointment after failure to add the FA Cup to the Carling Cup and not mounting a challenge to reach the Champions League places. May 5, 2012, Phil McNulty, “Chelsea 2-1 Liverpool”, in BBC Sport
   Since the launch early last year of […] two Silicon Valley start-ups offering free education through MOOCs, massive open online courses, the ivory towers of academia have been shaken to their foundations. University brands built in some cases over centuries have been forced to contemplate the possibility that information technology will rapidly make their existing business model obsolete. 2013-07-20, “The attack of the MOOCs”, in The Economist, volume 408, number 8845
2. Belonging to someone (one person, singular).
   Place the casualty on their back with feet and legs raised—this is called the shock position. [emphasis in original] Once the casualty is positioned, cover them to preserve body heat, but do not overheat. 2006, St. John Ambulance, First on the Scene: Student Reference Guide, Lesson 2, page 3
   I prefer to think that birds have a sufficiently developed sense of humour to enjoy the spectacle of a human being hunched beneath a bush kissing the back of their hand. 1980, Bill Oddie, Bill Oddie's Little Black Bird Book, page 112
   ‘I mean ... if somebody made a mistake,’ Harry went on, ‘and let something slip, I know they didn’t mean to do it. It’s not their fault,’ he repeated, again a little louder than he would usually have spoken. 2007, J. K. Rowling, Harry Potter and the Deathly Hallows, (quoted edition: London: Bloomsbury, 2008, page 93)

adv

1. Misspelling of there.