proximo

[prok-suh-moh] /ˈprɒk səˌmoʊ/
adverb
1.
in, of, or during the next month:
on the 10th proximo.
Compare instant (def 11), ultimo.
Origin
1850-55; < Latin proximō ablative of proximus next. See proximal
British Dictionary definitions for proximo

proximo

/ˈprɒksɪməʊ/
adverb
1.
(esp abbreviated in formal correspondence) in or during the next or coming month: a letter of the seventh proximo, prox Compare instant, ultimo
Word Origin
C19: from Latin: in or on the next, from proximus next