[A]pplied to succession when divided so as to give the representatives belonging to one branch the share only that their head or ancestor would have taken had he survived. Thus, in a gift to A and the children of B, if they are to take per capita, each child will have a share equal to that of A; but if they are to take per stirpes, A will take one half and the other half will be divided among the children of B. [Century Dictionary]
1797, contraction of and per se and, meaning "(the character) '&' by itself is 'and' " (a hybrid phrase, partly in Latin, partly in English). An earlier form of it was colloquial ampassy (1706). The distinction is to avoid confusion with & in such formations as &c., a once common way of writing etc. (the et in et cetera is Latin for "and"). The letters a, I, and o also formerly (15c.-16c.) were written a per se, etc., especially when standing alone as words.
The symbol is based on the Latin word et "and," and comes from an old Roman system of shorthand signs (ligatures) attested in Pompeiian graffiti, and not (as sometimes stated) from the Tironian Notes, which was a different form of shorthand, probably invented by Cicero's companion Marcus Tullius Tiro, which used a different symbol, something like a reversed capital gamma, to indicate et. This Tironian symbol was maintained by some medieval scribes, including Anglo-Saxon chroniclers, who sprinkled their works with a symbol like a numeral 7 to indicate the word and.
In old schoolbooks the ampersand was printed at the end of the alphabet and thus by 1880s the word ampersand had acquired a slang sense of "posterior, rear end, hindquarters."
"by, for," mid-13c., from Old French par, per, from Latin per (see per). It figures in some French phrases borrowed into English and in the formation of some words (parboil, pardon, parvenu). In some older borrowings from French it has been re-Latinized to per- (perceive, perfect, perform, pertain).