The IPC 1860 was replaced by the BNS 2023 (in force 2024-07-01); no verified section mapping on file for Section 43 — confirm the corresponding provision (savings: Section 358, BNS 2023).
The word "illegal" is applicable to everything which is an offence or which is prohibited by law, or which furnishes ground for a civil action; and a person is said to be "legally bound to do" whatever it is illegal in him to omit.