The origin of the Kayasths is discussed in the article on that caste.

Members of the above castes usually wear the sacred thread which is the mark of the Dwija or twice-born, the old Brahmans, Kshatriyas and Vaishyas.

The thread is not worn generally by the castes of the second group, but the more wealthy and prominent sections of them frequently assume it.
20.

Castes from whom a Brahman can take water.

Higher agriculturists.
The second group of good castes from whom a Brahman can take water falls into three sections as already explained: the higher agricultural castes, the higher artisans, and the serving or menial castes from whom a Brahman takes water from motives of convenience.