A short way of writing 'though'. Same word, same meaning, same sound — only the spelling is cut down. The apostrophe is standing in for the letters 'ugh' that have been dropped. Because 'though' means several things, so does this: 'but' or 'however' at the end of a sentence ('It's expensive. I want it tho.'); 'even if' or 'in spite of the fact that' at the start of a clause ('Tho' it was late, she kept working'). Where do you meet it? Two places, a hundred years apart. In old writing — nineteenth-century poems, letters, signs — writers used 'tho'' with the apostrophe to save ink and space, the same way they wrote 'thro'' for 'through'. Then it came back in our own time, in text messages and online, usually with no apostrophe at all: 'tho'. Young people write it constantly. There is even a common pattern, 'X tho', where you drop the thing you are arguing with: 'That price tho.' — meaning 'wow, look how high that price is'. Now the important part. This is informal writing only. Never write it in school work, an email to a teacher, a job application, or an exam. In speech there is no difference at all — 'tho' and 'though' sound exactly the same, so this is purely a matter of how it looks on the page.