windowsill: the sill of a window; the horizontal member at the bottom of the window frame
sill: structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure
rheumatoid arthritis: a chronic autoimmune disease with inflammation of the joints and marked deformities; something (possibly a virus) triggers an attack on the synovium by the immune system, which releases cytokines that stimulate an inflammatory reaction that can lead to the destruction of all components of the joint