odds-on

The word 'odds' itself originated in the late 15th century, derived from the earlier Middle English 'odde', meaning 'odd' or 'uneven', which came from Old Norse 'oddi', meaning a triangle or point of land, and thus a third or extra number beyond an even pair. This evolved into the concept of an unequal advantage or probability in wagering. The preposition 'on' is from Old English, indicating position or attachment. The compound 'odds-on' emerged in betting jargon of the mid-19th century, literally meaning that the 'odds' are 'on' a particular competitor, signifying it is the favorite. Over time, its use broadened beyond gambling to describe any outcome considered more likely than not, solidifying its status as a standard adjective.

The term is a fixed compound formed from the plural noun 'odds' (meaning the ratio between the amounts staked by the parties to a bet, based on the expected probability) and the preposition 'on'. It functions as a single lexical unit.