Is it ensure, insure, or assure?
1640s, from French prolifique (16c.), from Medieval Latin prolificus, from Latin proles "offspring" + root of facere "to make" (see factitious). Latin proles is contracted from *pro-oles, from PIE *pro-al-, from *pro- "forth" (see pro-) + *al- "to grow, nourish" (see old). Related: Prolifical (c.1600).
Prolific is in common use, but to make a satisfactory noun from it has passed the wit of man. [Fowler]