The compound "spent-fuel" is a modern technical term whose logic is transparent from its constituent parts. "Spent" originates from the Latin 'expendere', meaning to weigh out or pay, evolving through Old English to signify something that has been fully used or consumed. "Fuel" traces back to Latin 'focus', meaning hearth or fireplace, referring to material burned for energy. Combined in the mid-20th century with the advent of nuclear power, the term literally means "used-up energy material," directly describing reactor fuel that has been depleted of its fissile isotopes but retains dangerous radioactive byproducts.

Structure: spent (used up) + fuel (material for producing energy) - spent: From Middle English, past participle of 'spenden' (to spend, use up). Origin: Old English 'spendan', from Latin 'expendere' (to weigh out, pay). Functional role: Adjective/participle modifying the noun. - fuel: From Old French 'fouaille', from Medieval Latin 'focalia', from Latin 'focus' (hearth, fireplace). Functional role: Head noun.