This is a very casual, often regional way to say one when you mean a person or a thing of a certain kind: a good 'un, a bad 'un, their young 'un. It always sits after a describing word.

This這是 when you mean當你指 a person or a thing of a certain kind某一種人或東西時，to say one說one a very casual, often regional way非常隨便、常帶地方色彩的方式：a good 'un好傢伙、a bad 'un壞傢伙、their young 'un他們家的小傢伙。It它 always總是 sits坐 after a describing word在一個形容的字後面

Teachers and careful speakers do not use it. You will see it in stories that try to catch how someone from a place really talks.

Teachers老師 and careful speakers和講話講究的人 do not use it不用它。You你 in stories在那些 that try to catch想抓住 how someone from a place really talks某個地方的人實際怎麼講話 的故事裡 will see it會看到它

Though actually, the apostrophe is a bit of a fib. This is not one with a letter taken off. Old English said one without a w sound at the front, more like “un”, and this spelling keeps that old sound. So it looks like a short one, but it is really an old pronunciation wearing an apostrophe.

Though actually不過其實，the apostrophe那個撇號 is a bit of a fib有點在說小謊。This這 is not不是 with a letter taken off被拿掉一個字母的 one。Old English古英語 said one說one時 without a w sound at the front前面沒有w的音，more like更像 "un"，and this spelling而這個拼法 keeps保住了 that old sound那個古老的發音。So所以 it它 looks like看起來像 a short one縮短的one，but it really但其實是 wearing an apostrophe戴著撇號的 an old pronunciation一個古老發音