I mean an extremist.
"Youth and the Bright Medusa"
Willa Cather
He sat in the Long Parliament, and while an advocate of reform he was not an extremist.
"A Budget of Paradoxes, Volume II (of II)"
Augustus de Morgan
These unfortunate creatures began soon after midnight singing and shouting so as to avail themselves to the extremist limit of the holiday, which released them for a short time from their tasks and duties; Pollux knew well how unbounded the license of their pleasures could be, and as he walked on with Arsinoe he enjoined her to keep with him as close as possible to the houses.
"The Complete Historical Romances of Georg Ebers"
Georg Ebers