The term 'throw-in' is a transparent compound formed from two native English elements. The verb 'throw' has a deep Germanic origin, deriving from a Proto-Indo-European root meaning 'to twist' or 'to turn', which evolved into the sense of propelling something through the air with a motion of the arm. The particle 'in' is a fundamental preposition and adverb of location. Combined, they create a literal description of the action: throwing the ball into the field of play from outside its boundaries. This specific sporting sense emerged in the 19th century as the codified rules of games like soccer and rugby required a standardized term for this method of restarting play. The secondary, more figurative meaning of 'an added extra or interruption' logically extends from this core idea of something being introduced into a situation from the outside.

Structure: throw (to propel through the air) + in (into a place or condition) throw: From Old English þrāwan (to twist, turn). In this compound, it functions as the core verb indicating the action of propulsion. in: From Old English in (in, into). In this compound, it functions as a particle/adverb specifying the direction or result of the action.