rater

the word 'rater' is a modern english agent noun formed from the verb 'rate'. the verb 'rate' entered english from old french rater, meaning 'to estimate', which itself derives from medieval latin ratare, a frequentative of latin ratus, the past participle of rērī 'to reckon, think'. the core notion of 'reckoning' or 'calculating a value' has persisted. the agent suffix '-er', of germanic origin, is productively attached to verbs to denote a person or instrument performing the action, thus a 'rater' is literally 'one who reckons or assigns a value'.

Structure: rate (assess) + er (agent suffix) rate (from latin rata, from ratus 'reckoned, fixed', meaning 'to estimate or assign a value'); er (from old english -ere, an agent noun suffix indicating 'one who does something').