not easily come by; won through great effort or struggle

A perfectly sound proposition or irrefutable argument that cannot be changed or improved upon.

(literary) it is not easy to live in the capital; a metaphor for the high cost of living in a large city. This expression originates from a pun on the name of the Tang poet [[白居易|白居易|bai2 ju1 yi4]], to whom it was said that living in the capital Cháng'ān was not easy (bù yì).

really not easy; quite difficult to achieve or accomplish

fixed and unalterable; set in stone; remaining the same forever

not easily come by; difficult to obtain or achieve; earned through significant effort