The terms ‘heuristics’ and ‘rule of thumb’ are often used as synonyms, but there are some differences in meaning.(1) A rule of thumb was explained as a rough practical rule that was based on practice and experience. Most likely the saying comes from the length of the first joint of the thumb, which is about one inch.

(1) Simpson J, Wein E. Oxford English Dictionary.  2005. Oxford, Oxford University Press.