It is a pants-waist or vest-waist strap adjuster buckle. See the photos below.
If yours is unmarked, there's no way to time-date it, because they start showing up in the mid-1800s and are still being made today. That being said, the earlier ones were marked with either a date or maker's-name, so if yours is totally unmarked it is probably from the early-1900s, or perhaps later.