Apparently it’s an alternative form of “full rein.”
AHD: give free (or full) rein to To release from restraints
M-W: opportunity for unhampered activity or use <gave full rein to her imagination>
Quotes from Google Books:
Metamorphoses - Page 199
by Ovid, Charles Martin - Poetry - 2004
... who, having heard the clattering of arrows in a quiver, at once gave full rein
to his horse, fleeing as when a helmsman sees a distant cloud
The Man in the Iron Mask - Page 321
by Alexandre Dumas, Joachim Neugroschel, Francine Du Plessix Gray - Fiction - 2003
D’Artagnan gave his black horse full rein and it galloped away. Both men were
taking the same route. The quadruple echoes of the hooves mingled together.
The trees and fields went the other way - Page 32
by Evelyn Sybil Mary Eaton - Literary Criticism - 1974
The first time “giving full rein to his passion” appeared in print, it must have
seemed the ultimate image of frenzied urgency.