Dictionary.net

Civil law. So called from the word adsidere, which Signifies to be seated with the judge. They were lawyers who were appointed to assist, by their advice, the Roman magistrates, who were generally ignorant of law. being mere military men. Dig. lib. 1, t. 22; Code, lib. 1, t. 51.

In our law an assessor is one who has been legally appointed to value and appraise property, generally. with a view of laying a tax on it.