I poked around myself and the earliest use of the term I can find is in an article from 1968:
Pupil size and problem solving
JL Bradshaw
The Quarterly Journal of Experimental Psychology, 1968
Of course when I tried to read the text of the article, I got a message saying
Sorry, you do not have access to this article.
How to gain access:
Recommend to your librarian that your institution
purchase access to this publication.
If you search that book for "cognitive load is defined" you get the snippet:
> "Cognitive load" is defined loosely as the amount of mental strain put on a person during the performance of some task, often at least partially due to the constraints placed on the performance of that task...
The related term "mental load" seems to be a few years older, coming mostly from precursors to HCI, such as industrial ergonomics. Here's a use from 1961, also paywalled, but with the abstract available: http://www.tandfonline.com/doi/abs/10.1080/00140136108930502
"Pupil size changes were monitored during the solution of various types of problems. A number of solution and response strategies were required of the subject. There was strong confirmation of the theory that this autonomic index can provide a sensitive measure of the fluctuating levels of attention and arousal, which are associated with the various aspects of information processing and response."
