What is "costislost"

Costislost is a Grade II listed house in the parish of Egloshayle in Cornwall, England. To the south are Costislost Plantations. It dates to probably the 17th century, and underwent remodelling in the 18th and 19th centuries. It has an asbestos slate roof with gable ends. A farmhouse for centuries, it is now run as a bed and breakfast. It belonged to the Lakeman family in the 19th century.