وجب

Etymology 1
verb from the root.

Verb

 * 1)  to be necessary, have to be; must:
 * 2)  to be one's duty; to be incumbent, imposed, enjoined:
 * 1)  to be one's duty; to be incumbent, imposed, enjoined:
 * 1)  to be one's duty; to be incumbent, imposed, enjoined:

Usage notes
In the first sense, is impersonal: it does not take a personal subject. The person for whom something is necessary is given as the object of or as the subject of the verb in the clause introduced by the subjunctive particle.

Etymology 2
Causative verb from the root ; compare.

Verb

 * 1)  to impose  on

Noun

 * 1) span