Ajar comes from Middle English a- ('on') + char ('turn'), as on char ('on the turn'), contracted to achar (14th c., 'slightly open'); into Early Modern English ajar ('partly open'), giving Modern English ajar ('slightly open'). In current use it is an adjective meaning "slightly open or not completely shut, often of a door or window". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The surface form is compositional: its joined elements remain visible in the modern word. The word's history is thus visible in its construction: the components remain interpretable even after the whole expression has settled into a conventional English meaning.