Some English speakers use the word kudo as the singular of kudos. What makes this usage problematic is the fact that kudos is already singular.
Kudos is a Greek word meaning “glory, fame, renown.” It entered the language as student slang back when undergraduates were still required to study Greek at the university. Presumably the early users knew that it was a singular noun.
The earliest OED citation for the use of the back-formation kudo is dated 1941. The OED marks the use of singular kudo as “erroneous,” but Merriam-Webster provides kudo with its own entry, taking care to defend its position in doing so:
Some commentators hold that since kudos is a singular word it cannot be used as a plural and that the word kudo is impossible. But kudo does exist…
M-W’s assertion that kudo “does exist” makes me think of the comment made by Florence Foster Jenkins (1868-1944) about her excruciating efforts at singing opera: “People may say I can’t sing, but no one can ever say I didn’t sing.”
Inarguably, kudo is a word.
Modern English is filled with words that began as errors only to become perfectly acceptable standard words.
For example, our words newt and apron are the result of confusion over the indefinite article. What we now call “a newt” used to be “an ewt,” but the n of the article became attached to the noun. Conversely, what we call “an apron” started out as “a napron.”
In Chaucer’s day, what we call a pea was called a pease. The plural was pesen. By the 1600s, pease was viewed as a word that, like sheep, could be either singular or plural. Before the end of the 17th century, pease had become pea in the singular and peas in the plural. That the older form persisted for a time is indicated by the nursery song “Pease Porridge Hot,” which dates from about 1765. M-W cites pea in its defense of singular kudo.
Here are some examples of singular kudo on the web:
That deserves a big KUDO! (agricultural site)
Riverfront venue kudo deserved (Mankato Free Press)
That deserves an even bigger KUDO. (product testimonial)
How can I give a kudo to a great comment? (Myspace FAQ)
In a way, kudo is like pea; both are back-formations. But the changes in pease and pesen occurred at a time when other number changes were taking place. English speakers once formed the plural of hose as hosen and tree as treen. I can think of only two nouns that have kept the -en plural: child/children, ox/oxen. We still use the plural brethren in a spiritual sense, but the regular plural of brother is brothers.
It seems to me that kudo belongs with jocular back-formations like kempt from unkempt and gruntled from disgruntled. In these days of universal education and easy access to reference materials, using kudo seriously doesn’t seem any more acceptable than rendering the word as these writers have:
Jane Hamsher deserves Kudo’s (political blogger)
Director Brown reported the F&B Dept deserves a BIG KUDO’s. (minutes of a public meeting)
Kudo’s from clients (category on a technology site)