والواجب: هو السقوط
One linguistic sense associated with wajib is falling or becoming due.
Another explanation links the term to wajbah, a sense of disturbance or being between positions. This figuratively explains the special technical position of wajib between fard and nafl; it does not make performance of wajib optional.
Its technical classification must be established from the relevant proof and legal consequence, not merely from a linguistic etymology. Those consequences clarify what wajib actually requires.