The compound adjective 'three-stroke' is a transparent modern English formation, combining the cardinal number 'three' with the noun 'stroke'. The word 'three' descends from the Proto-Indo-European root trei-, which denoted the quantity and is the source for the number across many Indo-European languages. 'Stroke' has a dual lineage in English, relating both to gentle touch and forceful blow, with the latter sense deriving from the concept of a single delivered action. When combined, the morphemes create a precise technical descriptor, logically specifying a quantity of actions, most notably used to classify a type of internal combustion engine based on its three-piston-stroke cycle, in contrast to the more common two-stroke or four-stroke designs.

Structure: three (number) + stroke (act of striking) - three: From Old English þrīe, þrēo, of Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as a numerical quantifier. - stroke: From Old English strācian (to caress) and strāc (a blow), of Germanic origin. In this compound, it functions as a noun denoting a single complete movement or impact.