equanimous

Definitions:

Of an even, composed frame of mind; of a steady temper.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)

Usage examples:

Being of a pretty equanimous disposition, and, as I flatter myself, able to bear good or evil fortune without disturbance, I myself, after taking a light dinner at the Bedford, went to the theatre a short while before the commencement of the play, and proposed to remain there, until the defeat or victory was decided.
- The Virginians by William Makepeace Thackeray