The story
The x functions as a wildcard; the processors immediately succeeding the original 8086 and using the same architecture (with progressively greater expansions) also had names ending in 86 (the 80186, 80286, 80386, and 80486; Intel shifted away from this numerical nomenclature after the 80486, but other manufacturers of x86 processors continued the series with -586- and -686-branded processors).
Etymology from Wiktionary, CC BY-SA 4.0.