How did bastard files get their name?
With 'smooth' files being at one end of the coarseness scale for American pattern files, the name 'bastard' seems a little out of place to describe one of the roughest types, so how did the file get its name?
Bastard files: betwixt and between
There is a theory that the term 'bastard' is used to describe files that are neither coarse nor smooth, and therefore the odd one out. This has loose ties to the word's meaning in the modern day.
A more likely explanation...
Another theory is that files earned this nickname in medieval England (the term 'American pattern' was only given to files many years later).
In heraldry, coats of arms belonging to people born outside of wedlock (also known as bastards) bore a device known as the 'barre sinister', a diagonal stripe that ran from the top right of the crest to the bottom left.
This is the same direction that the teeth of a single cut file run in.
Therefore, it seems most likely that this was a humorous nickname that has stuck, and remains in use today as a technical term!