means The quality of being a distinct, continuous individual self — your sense of who you fundamentally are.

from A transparent compound: the Old English self (one's own person, going back to a Germanic root shared with German selbst) plus the suffix -hood, from Old English -hād, which once meant 'state, condition, or rank' and survives in childhood, knighthood, and brotherhood. So selfhood is literally 'the state of being a self' — though as an abstract noun it's a relative latecomer, emerging only once philosophers and writers needed a word for the bare fact of being someone.