[zeckalpha]

i came from imaginary numbers which were extended to make quaternions.

[jibal]

i, j, k comes from FORTRAN's implicit types -- by default, names starting with I-N are integers and all other names are real.

[jcelerier]

this is much older ; Joseph Fourier was already using "i" and "j" for indices in the 1800s. See page 209: https://www.google.ca/books/edition/OEuvres_de_Fourier_Th%C3...

[jibal]

The context is i, j, k as indices in programs. No doubt FORTRAN was influenced by prior use such as you cite. But in no case does i used as an index come from i designating an imaginary number, which is what I aimed to refute.

[zeckalpha]

Representing dimensions as indexable rows and columns of vectors or matrices was done on paper in the 1800s.