silny (Old Polish) comes from Proto-Slavic silьnъ, from Proto-Slavic sila, from Proto-Balto-Slavic séiˀlāˀ, from Proto-Indo-European seh₂i-l-eh₂, from Proto-Indo-European sh₂ey-, from Proto-Indo-European seh₂- — to satiate, satisfy.
silny (Old Polish): strong