From Latin 'assidēre' (to sit beside) — originally sitting beside a judge as a tax-determining assistant, shifted to the act of evaluating.
To evaluate or estimate the nature, ability, or quality of; to determine the value of something for taxation; to impose a tax or charge.
From Medieval Latin 'assessāre' (to fix a tax), frequentative of Latin 'assessus,' past participle of 'assidēre' (to sit beside, to assist a judge), composed of 'ad-' (to, beside) + 'sedēre' (to sit), from PIE *sed- (to sit). The original sense was 'sitting beside' a judge as an assistant — the assessor helped determine the amount of a tax or fine. The word moved from the assistant's action (sitting beside) to the assistant's function (evaluating). Key
The word 'assiduous' (hardworking, persistent) is from the same Latin verb — 'assidēre' (to sit beside). An assiduous person is one who 'sits beside' their work and does not leave — a metaphor of persistent seated attention. Similarly, 'assize' (a court session, especially for setting standards)