«الواجب: من الوجوب، وهو السقوط، وشرعاً: ما ثبت بدليل فيه شبهة»
Lexically, wajib is associated with falling into place or becoming binding; technically it is a duty established through evidence with an element of non-conclusiveness.
Here shubhah does not mean a baseless suspicion or an unreliable guess. It indicates that either the proof or its legal indication does not reach the decisiveness associated with fard, despite remaining authoritative for action.
A rule can therefore be practically binding even though its evidentiary grade differs from that of fard.