I expect that many of us can recount times when we have entered wrong orders into the market. Perhaps when you enter a buy order, but you meant to sell. Maybe you accidentally put a zero on the end of you lot size and only realised later? Well, the phrase 'fat finger' is simply a colloquial way of saying, someone has made a mistake. It is said to be a fat finger. In many ways I quite like the phrase as it depersonalises what can be a very embarrassing and costly mistake. Saying a 'fat finger' is to blame is much better than saying , oh it was Mr Joe Bloggs who works for BNP Paribas, yes, he is the man who brought a stock to it's knees overnight. He was thinking about the Man U vs PSG coming up you see and just switched off for a second.
Some of the moves cause by 'fat fingers' can be large. In April 2018 Deutsche Bank AG accidentally transferee 28 billion euros to an outside account as part of its daily derivative dealings. Ouch. One of outhitting Korea's target brokerages was trying to pay their employees 88 cents per share in dividends in April 2018. However, instead it gave them 1000 shares instead. That totalled about $105 billion, which was thirty times more than the company was worth. The kicker came when 16 employees sold their stock and this created a huge fall in the share price. They must have thought it was their lucky day.
Now, in many cases these errors can be reversed. However, it is hard to eradicate these types of human errors. Germany's chief financial regulator noted in March that the majority of computer security problems were caused, not by cybercriminals, but by human 'mishaps or mistakes'. See here for a gated link.
So, with lots of transactions on a daily basis, fat fingers are here to stay. If you want to look at a strategy to avoid them yourself, then check out Adam's post here and the video at the bottom.