1911 (adj.) "having average or moderate cultural interest;" 1912 (n.) "person of average or moderate cultural interests," from middle (adj.) + brow (compare highbrow, lowbrow).

[T]here is an alarmingly wide chasm, I might almost say a vacuum, between the high-brow, who considers reading either as a trade or as a form of intellectual wrestling, and the low-brow, who is merely seeking for gross thrills. It is to be hoped that culture will soon be democratized through some less conventional system of education, giving rise to a new type that might be called the middle-brow, who will consider books as a source of intellectual enjoyment. ["The Nation," Jan, 25, 1912]

updated on January 14, 2019

Definitions of middlebrow from WordNet

middlebrow (n.)
someone who is neither a highbrow nor a lowbrow;
Etymologies are not definitions. From, not affiliated with etymonline.