Ultimo means last month. Together with instant and proximo it’s an example of an outdated commercial language. Few businessmen would today begin a letter “With reference to yours of the 14th ultimo”, or “yours of the 23rd instant”, or “Please attend this office for interview on the 11th proximo”, but it was once standard and taught in the best books. All three were commonly abbreviated, to ult, inst and prox respectively.
Ultimo and proximo are both Latin, shortened forms of ultimo mense, in the previous month, and proximo mense, in the next month. Many reference works say inst is from Latin instante mense, in the current month. But the Oxford English Dictionary points out that it has always been expanded to the English word instant, in the specialised meaning of current.
By 1922, such terms were being satirised in Punch:
Bear up, brave clerklets, though the lights of learning
Your quaint commercial English sadly shocks,
And even your bosses are agreed in spurning
Your “inst”, and “ult”, and “prox”.
I like the pleasant jargon: I should miss it
If firms no more (“per pro” before their name)
Should “thank me for past favours and solicit
Continuance of the same”.