What is a verbal noun; or (according to C.F.Becker), a case of the infinitive mood ending in -um and -u, that in -um being sometimes called the former supine, and that in -u the latter supine called?

supine — A verbal noun; or (according to C.F.Becker), a case of the infinitive mood ending in -um and -u, that in -um being sometimes called the former supine, and that in -u the latter supine. (Webster's 1913.)

Sources