Roughly, the word "heuristic", or heuristic rule, was a rule that was approximate … basically, a rule of thumb.

Dictionary definition, although it is a bit opaque:
Of or relating to a usually speculative formulation serving as a guide in the investigation or solution of a problem: "the historian discovers the past by the judicious use of such a heuristic device as the ‘ideal type’ " (Karl J. Weintraub).

The etymology is a bit more transparent:
< Gk. heuriskein, to find.
Heuristic, or heuretic, or "ars inveniendi" was the name of a certain branch of study, not very clearly circumscribed, belonging to logic, or to philosophy, or to psychology, often outlined, seldom presented in detail, and as good as forgotten today. The aim of heuristic is to study the methods and rules of discovery and invention.
Heuristic reasoning is reasoning not regarded as final and strict but as provisional and plausible only, whose purpose is to discover the solution of the present problem. We are often obliged to use heuristic reasoning. We shall attain complete certainty when we shall have obtained the complete solution, but before obtaining certainty we must often be satisfied with a more or less plausible guess.
In that last sentence, you can see the "rule of thumb" aspect picked up on.

Bolzano, Bernard (1781–1848), logician and mathematician, devoted an extensive part of his comprehensive presentation of logic, Wissenschaftslehre, to the subject of heuristic (vol. 3, pp. 293–575). He writes about this part of his work: "I do not think at all that I am able to present here any procedure of investigation that was not perceived long ago by all men of talent; and I do not promise at all that you can find here anything quite new of this kind. But I shall take pains to state in clear words the rules and ways of investigation which are followed by all able men, who in most cases are not even conscious of following them. Although I am free from the illusion that I shall fully succeed even in doing this, I still hope that the little that is presented here may please some people and have some application afterwards."
This conveys the idea of collecting good ideas from all sources while acknowledging that there's nothing new under the sun … not even the idea of collecting good ideas, apparently.