also: こしかた
noun
Literary or poetic term for the past, often with a nostalgic or reflective tone. Not used in everyday conversation.
来し方を振り返ると、いろいろなことがあった。
Looking back on the past, many things have happened.
彼は来し方の思い出に浸っていた。
He was lost in memories of bygone days.
過去 is the standard, neutral word for 'the past' in modern Japanese. 来し方 is a literary, emotive alternative.
Compound of 来し (kishi, classical attributive form of 来る 'to come') + 方 (kata, 'direction, side'), literally 'the direction from which one has come', hence 'the past'. The reading こしかた uses the classical attributive form こし.