Kudos is originally a Greek word. In ancient Greek 'kydos' is a singular noun meaning praise, fame or renown. In Greek it is very common for singular nouns to end in 's' - plurals, especially of masculine nouns, are often 'oi'.

English speakers began to use the word kudos in the 19th century in its anglicised form - before that, it was used in the original Greek, mainly only by scholars of ancient languages. It has kept roughly the same meaning since then, but a lot of people make a mistake in its usage; because of its 's' ending, many people think it is a plural noun, so you get sentences like 'he received many kudos for his achievements'. This is wrong; it should be 'much kudos' or another singular form. There is also a tendency to use the work 'kudos' as a synonym for 'award' or 'prize'. This is also wrong; kudos is honour, praise etc, not a tangible reward.