Usage Discussion of tho
While never extremely common, tho and thru have a long history of occasional use as spelling variants of though and through. Their greatest popularity occurred in the late 19th and early 20th centuries, when their adoption was advocated by spelling reformers. Their current use occurs chiefly in informal writing (as in personal letters) and in some technical journals.
Seen and Heard
What made you want to look up tho? Please tell us where you read or heard it (including the quote, if possible).