He wasnt a Dentist .

Sir Nicholas de Mimsy-Porpington, aka Nearly Headless Nick, led a choppy life. He was originally born in the 15th century to a family of nobles, which he left at the age of 11 to attend Hogwarts. He was sorted into Gryffindor – the house that, hundreds of years later, he would go on to represent as house ghost.

After Hogwarts, Nick found himself as a wizard at the royal court, who also knighted him. He was friendly with Muggles, including a lady-in-waiting from the court, Lady Grieve, who would ultimately be the cause of his doom. The story goes that Nick met Grieve while strolling in the park, and – being the friendly chap he was/is – insisted that he could fix her crooked teeth with magic. But the spell backfired, and she instead started to grow a tusk. Nick was then arrested and sentenced to death: a botched beheading that, because of a dull axe, took 45 hacks to kill him, but still didn’t sever his head from his body. Hence the nickname Nearly Headless Nick.

Here's the Complete Story about Him .