The following extract from the Oxford English Dictionary, art. "plus", is perhaps of interest:
The prepositional use (sense A. 1), from which all the other English
uses developed, did not exist in Latin of any period. It probably
originated in the commercial language of the Middle Ages (see
discussion at minus prep., n., adv., and adj.). The signs + and −
occur for the first time in print in J. Widmann Behende vnd Hubsche
Rechenung auff allen Kauffmanschafft (1489), a treatise on commercial
arithmetic; but while Widmann refers to the latter as minus , the
former is said to stand for mer (i.e. mehr ): ‘was − ist, das ist
minus, und das + ist das mer’. So far as is known, English examples of
plus do not occur as early as those of minus .
In a somewhat different sense, Latin plus and minus had been employed
in 1202 by Leonardo Pisano (also known as Leonardo Fibonacci) for the
excess and deficiency in the results of the two suppositions in the
Rule of Double Position (see position n. 2); see further M. Cantor
Vorlesungen über Geschichte der Mathematik (ed. 2, 1899) II. 230–1.
As to the origin of the symbol +, the most likely is that it
originated as one of the MS abbreviations of the Latin et ‘and’ (see
F. Cajori Hist. Math. Notations (1928) I. iii. 229–39).
Instances of the written word (as opposed to the symbol) are rare
before the 17th cent.
The article "minus" says more or less the same.