1) Originally the catkin of trees such as the willow and hazel, linked with ‘kitten’ via its downy appearance (OED).
In that sense it is on record from c.1400 but when used of the seed or ‘key’ of other trees it dates only from 1562. Sir Henry Cholmley used the word frequently in his memorandum book: 1653-4 I did sowe … Ash chats … on the Ditch on the Moore
1667 I planted the trees at the west end of the coalecoate Ash by sowing Ackornes and Sycamore chatts, Oswaldkirk. The reference to sowing chats in ditches strengthens the possible use of the word as a place-name element: 1544-5 Bosc vocat. Chatpitt sprynge, Wragby
1592 William Broadley of Chates, Hunsworth. See acorn, sycamore.