This is the 9729th most frequent Russian word.
шлеменко
"Шлеменко" is a proper noun, typically a surname in Russian, and does not have a specific definition in English beyond being a name.
In this sentence, 'Шлеменко' is used as a proper noun, likely referring to a person's last name.
Шлеменко готовится к важному поединку.
Shlemenko is preparing for an important fight.
Here, 'шлемёнок' is derived from 'шлем' (helmet) with a diminutive suffix, personified as a mysterious item.
В музее был найден древний шлем, но откуда произошёл шлемёнок, никто не знал.
In the museum, an ancient helmet was found, but where the young helmet came from, no one knew.
In this use case, 'шлем' straightforwardly refers to a 'helmet' as an object.