In the 19th century, kudos1 entered English as a singular noun, a transliteration of a Greek singular noun kŷdos meaning “praise or renown.” It was at first used largely in academic circles, but it gained wider currency in the 1920s in journalistic use, particularly in headlines: Playwright receives kudos. Kudos given to track record breakers. Kudos is often used, as in these examples, in contexts that do not clearly indicate whether it is singular or plural; and because it ends in -s, the marker of regular plurals in English, kudos has come to be widely regarded and used as a plural noun meaning “accolades” rather than as a singular mass noun meaning “honor or glory.”
The singular form kudo has been produced from kudos by back formation, the same process that gave us the singular pea from pease, originally both singular and plural, sherry from Xeres (an earlier spelling of the Spanish city Jerez), and cherry from the French singular noun cherise. This singular form has developed the meanings “honor” and “statement of praise, accolade.”
Both the singular form kudo and kudos as a plural are today most common in journalistic writing. Some usage guides warn against using them.