one-track

The compound 'one-track' is a modern English formation, drawing from two deeply rooted Germanic words. 'One' originates from Proto-Germanic 'ainaz', which itself descends from the Proto-Indo-European root 'óynos', meaning 'single, one'. 'Track' comes from a Middle Dutch or Middle Low German word for a beaten path, likely related to the act of dragging or drawing, tracing back to Proto-Germanic 'trak-'. The metaphorical sense emerged in the late 19th to early 20th century, directly from the image of a railway with only a single line of rails, which allows traffic in only one direction at a time. This literal constraint was logically extended to describe a mind or focus that is similarly limited to a single idea or course, unable to accommodate multiple thoughts or interests simultaneously.

Structure: one (single) + track (path) - one: From Old English 'ān', meaning 'single, one'. In this compound, it functions as a quantifier meaning 'single' or 'sole'. - track: From Middle English 'trak', meaning 'path, course'. In this compound, it functions as a noun representing a literal or figurative path or line of thought.