Definition of "ultimo"

I receivd your favour of the 18th Ultimo in due time & beg pardon for not having answerd it before.

1771 November 20, Alexander Hamilton, “From Alexander Hamilton to Fraser, Grant and Baillie, 20 November 1771”, in Harold C. Syrett, editor, The Papers of Alexander Hamilton‎, volumes 1, 1768–1778, New York: Columbia University Press, published 1962, page 15