Deriving from Greek, kudos entered English as slang popular at British universities in the 19th century. In its earliest use, the word referred to the prestige or renown that one gained by having accomplished something noteworthy. The sense meaning "praise given for achievement" came about in the 1920s. As this later sense became the predominant one, some English speakers, unaware of the word's Greek origin, began to treat the word as a plural count noun, inevitably coming up with the back-formation kudo to refer to a single instance of praise. For the same reason, when kudos is used as a subject you may see it with either a singular or plural verb.