ārū

ārū has that –ū suffix, and it means 11 in base 8 or 9 in base 10. The stem ār is derived from an old word for 3, which is only ever found these days in derived words and in old poetry.

The other word for 11 is ōr aþān (10 and 1), but rather than count ōnne (7), ōrū (10), ōr aþān (11), ōr aþēnne (12) many people will count ōnne (7), ōrū (10), ārū (11), ārū aþān (11 and 1).