Possibly the first published use of the term in software was Van Wijngaarden, in the design of Algol 68, from 1968:
> The number of independent primitive concepts has been minimized in order that the language be easy to describe, to learn, and to implement. On the other hand, these concepts have been applied “orthogonally” in order to maximize the expressive power of the language while trying to avoid deleterious superfluities.
> The number of independent primitive concepts has been minimized in order that the language be easy to describe, to learn, and to implement. On the other hand, these concepts have been applied “orthogonally” in order to maximize the expressive power of the language while trying to avoid deleterious superfluities.
https://www.softwarepreservation.org/projects/ALGOL/report/A... , section 0.1.2, Orthogonality.