A literal or figurative voyager.
[Greek nautēs, a sailor.]
The first English word in this ending was Argonaut, from the Greek term for one of the legendary heroes who accompanied Jason in the Argo in his quest for the Golden Fleece. Later additions were aeronaut (see aer(o)-), a balloonist, which came into English from French in 1784, aquanaut (see aqua-), an underwater explorer or swimmer. Astronaut (see astr(o)-), a space traveller, was modelled on them in speculative writing in the 1920s, long before one actually existed.
With the development of space exploration, several terms have been created in imitation of astronaut in a spirit of national pride and identity. The first was the Russian cosmonaut (see cosmo-), followed by spationaut (from French spatial) the French term for a French astronaut, taikonaut (from Chinese tai kong, space, though the official Chinese word is yuhangyuan) and Indian vyomanaut (from the Sanskrit word for sky, vyoma).
Other terms modelled on these include cybernaut (see cyber-), for an expert or habitual user of the Internet or someone who interacts with virtual reality by using computer technology; lunarnaut (Latin lūna, moon), an explorer of the moon; Marsnaut, similarly an explorer of Mars; oceanaut, another term for aquanaut; and psychonaut (see psych(o)-), a person who takes psychedelic drugs, mainly to investigate their effects.
Juggernaut, an overwhelming or unstoppable force or institution, is unconnected, the second element being from Sanskrit nātha, lord or protector. Jargonaut (from English jargon), a person who uses an excessive amount of jargon, appears from its sense to be from juggernaut rather than from -naut, though the suffix is likely to have influenced its formation.