expression
I ...; one ...
Archaic/literary expression where が marks the subject; equivalent to modern 自分が. Rare outside classical or highly stylized writing.
己が行く道を信じよ。
Believe in the path you yourself walk.
Modern standard word for 'oneself'; 己が is archaic and literary.
Also archaic/literary possessive, but more common in set phrases like 我が国.
From classical Japanese 己 (ono, 'self') + possessive/subject particle が. The form is fossilized and not productive in modern Japanese.