The word 'thankless' is a transparent compound formed within English from its native Germanic elements. Its core, 'thank', originates from the Old English þanc, which broadly meant "thought" but specifically evolved to signify "favorable thought, gratitude." The suffix '-less', from Old English -lēas, meant "devoid of." Therefore, the literal construction "devoid of thanks" perfectly captures the modern sense. The term first described tasks or roles that, despite requiring effort, fail to elicit gratitude. Over time, it was also applied to describe a person who is ungrateful, completing the semantic journey from a literal description of an action's reception to a characteristic of an individual.

Structure: thank (gratitude) + less (without) - thank: From Old English þanc (thought, good will, gratitude). It functions as the core noun root denoting gratitude or appreciation. - less: From Old English -lēas (devoid of, free from). It is a suffix used to form adjectives meaning "without" the noun it attaches to.