More firm, secure, or fixed together than something else; more steadfastly joined.
From co- (together) + faster (comparative of fast, from Old Norse fastr, meaning firm or secure). This archaic or rare adjective compares the degree to which things are firmly joined together.
This is one of those words that almost certainly appears in historical texts or specialized woodworking manuals, where craftsmen describe joints and fastenings with meticulous precision—cofaster would mean 'more tightly joined together,' a detail that matters when building something that needs to last centuries.
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