Historically, i wasn't even an abbreviation: It was the first variable name which would be assumed to be integer by FORTRAN compilers which implicitly assigned

Historically, i wasn't even an abbreviation: It was the first variable name which would be assumed to be integer by FORTRAN compilers which implicitly assigned types to variables based on name. The choice was probably further influenced by longstanding mathematical tradition, which uses i and j as indices. (You could declare types and the compiler would respect it, leading to the old truism "GOD is REAL, unless declared INTEGER".) (If you think that's the weirdest thing old FORTRAN did, look up the arithmetic IF statement sometime. Then, look up assigned GOTO.)