from the GNU version of the Collaborative International Dictionary of English
- adj. the solid foundation of coarse stuff, on which the rest is placed; it is thick, and crossed with lines, so as to give a bond for the next coat.
Sorry, no etymologies found.
I remember being assured by a person of veracity that he himself under an operation for the cataract, when a small silver needle was inserted within the first coat of the eye in order to remove the pellicle of the cataract and push it into a corner, saw most distinctly the needle passing over the very pupil.