Merthersitheny, Sanctus Sithinus, Merthersithun, St Sythyn, St Sythninus 1230, Sanctus Sydnius 1270, Merthersytheny 1284, Merdhersydny 1289, Sancto Sydeny 1291, Sanctus Suthninis 1302, Merthersithny 1304, Sancto Sythenyny 1310, Sancto Sidnino 1314, Merthersythny 1320, Sancto Sydyno 1342, Merthersetheny 1376, Sancto Siddinino 1379, Seynt Sitheny 1380, Sancto Sednino 1390, Sancto Sythnino 1403, 1432, Seyntsythny 1483, Sydny 1553, Seynt Synney 1554, Senye 1598, Egloseynney 1623.