What is the X in X-mas?
"unspectacular," 1909 in a literal sense, in reference to material yielded by a mill, etc., before sorting for quality (cf. common run "usual, ordinary type," from 1712). Figurative use is from 1922.
Common, ordinary, average: “His performance in the game was neither exemplary nor disastrous; it was simply run of the mill.”
Ordinary; average; garden-variety (1930+)