Stories We Like: Novels For Language Lovers
late 14c., "writer of commentaries," agent noun in Latin form from comment or commentary (Latin commentator meant "inventor, author"). Middle English also had a noun commentate, attested from early 15c. Meaning "writer of notes or expository comments" is from 1640s; sense of "one who gives commentary" (originally in sports) is from 1928.
"Well, Jem, what is a commentator?["]--"Why," was Jem's reply, "I suppose it must be the commonest of all taturs." ["Smart Sayings of Bright Children," collected by Howard Paul, 1886]