late Middle English: from Old French cohorte, or from Latin cohors, cohort- 'yard, retinue'. Compare with court
The co- in cohort is not a prefix signifying a joint or auxiliary relationship (as in coauthor or codependency). The word derives from the Latin cohors, an ancient Roman military unit, and also ‘band of people with a common interest.’ In the mid 20th century, a new sense developed in the US, meaning ‘a companion or colleague,’ as in young Jack arrived with three of his cohorts. Although this use is well established, there are still some people who object to it on the grounds that cohort should be used only for groups of people, never for individuals.