"overgrown with rushes," 1680s, from seave "rush" (c.1400), from Old Norse sef.
Online Etymology Dictionary, © 2010 Douglas Harper
Examples from the Web for seavy
Historical Examples of seavy
The proprietors, Messrs. Vail and Seavy, determined to bore for a spring.Saratoga and How to See It
R. F. Dearborn