Who first used the term 'heuristic' in a cognitive science context? I seem to recall that Herbert Simon borrowed the term from computer science, but I cannot remember the initial paper in which he made use of this borrowing.

A google scholar search reveals some papers from 1957-1958, for instance this one, but I really felt there was something from earlier in the 1950s.
A nice summary about heuristics can be found in: 

Gigerenzer, G., &amp; Gaissmaier, W. (2011). Heuristic Decision Making. Annual Review of Psychology, 62, 451–482.

In this review "satisficing" (accepting a good enough option) is referred to an earlier reference than the 1958 you cite:
Simon, H.A. (1955). A behavioral model of rational choice. Q. J. Econ. 69:99–118.

In their (Gigerenzer &amp; Gaissmaier) history part they also refer to Einstein using the term in his 1905 paper on quantum physics, stating that the view he presented was "incomplete but highly useful".