'assidual' originates from Latin, specifically the word 'assiduus', where the prefix/adfix elements relate to 'ad-' (to/toward) and the root 'sedēre'/'sedere' (to sit, to remain).

'assidual' developed from Latin 'assiduus' and related Late/Medieval Latin formations (e.g. 'assidūālis'), and entered English as a rare/analogous adjectival form parallel to 'assiduous' in early modern usage.

Initially it carried the sense of 'sitting near, attending constantly' (from the idea of remaining or sitting by), and over time this shifted to the modern sense of 'constant, diligent, persistent'.