The compound adjective "variable-speed" is a modern technical term formed by combining two English words of distinct origins. "Variable" descends from the Latin variare, meaning to change or diversify, which itself may trace back to the Proto-Indo-European root wer-, meaning to turn or bend. "Speed" originates from the Old English spēd, which initially meant success or prosperity before its meaning narrowed to focus on rapidity and rate of motion. The logic of the compound is transparently literal: it describes a mechanism whose rate of motion (speed) is changeable (variable), directly applying the core meanings of its constituent parts to specify a technical capability.

Structure: variable (changeable) + speed (rate of motion) - variable: From Latin variabilis (changeable), from variare (to change). An adjective-forming suffix indicating capability or tendency. - speed: From Old English spēd (success, prosperity, swiftness). A noun used in a compound to specify the type of capability.