The word 'unrewarding' is a modern English formation built upon the established verb 'reward'. The core 'reward' entered English via Anglo-Norman French, tracing back to the Old French reguarder, meaning 'to guard, watch over', which evolved to signify 'to take notice of' and thus 'to give something in recognition'. The negative prefix 'un-', of ancient Germanic lineage, is one of the most productive in English for creating opposites. The suffix '-ing' converts the verbal action into an adjective describing a quality. Therefore, the literal construction "not-giving-recognition" logically evolved into the modern abstract sense of an activity that fails to provide satisfaction or a worthwhile return for one's effort.

Structure: un (not) + reward (compensation) + ing (adjective suffix) - un: A prefix of Germanic origin (Old English un-), meaning "not." It functions as a negative particle. - reward: A noun/verb of North Germanic origin (Old French reguard, reguarder, from re- + guarder), meaning to regard, take notice of, or compensation. In this word, it functions as the core semantic base. - ing: A suffix of Germanic origin (Old English -ende, -ung), used here to form a participial adjective from the verb 'reward'.