ecclesie silin c.1191, Llansilyn 1254, Lansylyn 1291, Ecclesia Si Egidii de Kynlleith 1296, Llansilin 1419, ll. silyn yng hynllaith c.1566
The reference to St Egidius stems from a (probably mistaken) belief that Silin corresponds to Giles, of which Egidius is a latinised variant. The parish lay in the commote of Cynllaith which appears in two of the cited forms. Cynllaith is probably a personal name as in Machynlleth.