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Although dictionary etymologies are virtually nonexistent, a plausible origin can be extracted from historical usage. "Bastard file" in current usage is a file of coarse cut (as opposed to a "finishing" file. However, its technically precise definition is "a file one cut finer than a "coarse file." Files are classed as "coarse," "second cut" and "smooth," from coarsest to finest. Thus, a "bastard file" is a cut in between a "coarse" and a "second cut." The word "bastard" functions here in its meaning as "irregular." So, a "bastard file" is a file that is neither "coarse" nor "second cut."