from The Century Dictionary and Cyclopedia
- Pertaining to or of the nature of a gerundive. Also gerundial.
Sorry, no etymologies found.
This gerundival use of the infinitive is explained in K.S. 402 y and G.K. 114 o.
For these may be formed from the nouns laugh, avail, since - able is not only gerundival (capable of being laughed at), but also adjectival (connected with a laugh); this has certainly happened with seasonable; but that will not help reliable, which by analogy should be relianceable.