Wordorigins.org

This Americanism, meaning mediocre or of moderate quality, dates to the early 19th century. The following appeared in an ad in the Eastern Argus of Portland, Maine on 23 March 1824:

J. Haskell
Has just received 4 cases prime HATS, new style—also, 4 cases imitation HATS, at $2, “from fair to middling.”