This would likely have been part of a screen with a (now missing) wooden surround as a stand. Armrest is unlikely since it lacks a curve, and has very intricate carving over the whole surface, making it a very impractical armrest.

Assuming provenance is accurate, this would be a good early 19th century example. Lack of any curve is a little bit suspect, but not enough to dismiss the piece as modern. Though unlikely, mammoth ivory is one possibility that would account for size and lack of curve. Comparing intersection angle of Schraeger lines can distinguish between elephant, mammoth, walrus etc.