Vexed is the past participle of 'vex', from Old French 'vexer', from Latin 'vexare' ('to shake, harass'). The 'much-debated' sense reflects an issue that troubles or agitates. In current use it is an adjective meaning "annoyed or irritated; difficult and much debated". 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 history is mainly one of transmission into English, with the older form and sense carried through borrowing or adaptation. A central historical form is Latin vexare. Related English forms include vex, vexation, and vexatious. For dictionary purposes, the important point is the continuity between the older form, the route into English, and the present lexical sense.