the.com/fastening

means A device or method—like a clasp, button, zipper, or knot—used to join things together or hold something shut.

from From 'fasten,' rooted in Old English 'fæstnian,' meaning to make firm or secure, which traces back to 'fæst' — the same root behind the modern word 'fast,' as in 'stuck fast' or 'holding fast.' Oddly, that's the same core sense preserved in phrases like 'fast asleep': not speed, but fixity. The '-ening' simply turns the act of making-firm into a noun for the thing that does it.