Early-attested site in the Parish of North Petherwin
- Casteswurth 1228 FF
- Catteswurthi 1249 Ass
- Castesworthy 1270 Ass
- Keastesworthi 1330 SR
- Carsworthy 1551 DeedsEnrolled 1612 Recov
- Kersworthy al. Kearsworthy al. Catisworthy 1762 Recov
v. worþig . For the first element we may compare Keason (Co), Kestingtun c. 1150 (1348) Pat, Keyston 1571FF , and Lankeast (Co), Lankest 1364Ass , 1380 FF. Loth (Chrest. 19) notes Cast as the name of a Breton saint, possibly derived from the Lat castus . There seems to be no evidence that this was used as an ordinary pers. name in British, but it may possibly be connected with the Cornish surname Keast .