from opine + -ant,
possibly from French opinant (someone who gives an opinion in a deliberation)
“…The opinions differ pretty much according to the nature of the opinants. I say the Duke and Tom could have held out, that they meant to hold out, that they did hold out, and that there has been fistifying enough…”
From: Roundabout Papers.
On Some Late Great Victories
By William Makepeace Thackeray, 1860