дурак

Fool, idiot, blockhead. One of the most famous and fundamental Russian insults, meaning a stupid, foolish, or naive person. While it is technically a mild derogatory term, its cultural weight is immense, famously appearing as the archetype of the well-meaning underdog (Иванушка-дурачок) in Russian folklore. In contemporary speech, its severity depends entirely on tone — it can range from an affectionate tease between partners to a genuinely dismissive, sharp insult in a heated argument.

У старинушки три сына:
Старший умный был детина,
Средний сын и так и сяк,
Младший вовсе был дурак.

The old woman had three sons:
The eldest was a smart lad,
The middle one was so-so,
The youngest was a complete fool.

(П. Ершов, Конек-Горбунок)

Даня Саша дураки (“Danya and Sasha are idiots”),
Graffiti in Moscow, khokhlovskij pereulok (Feb. 2008)

To copulate (“thrush the idiot under the skin”).

Он безропотно освобождал комнату, когда у меня появлялось желание загнать кому-нибудь дурака под шкуру. Чаще всего шкура принадлежала Нине.

He would meekly clear the room when I felt the urge to drive one under someone’s skin. Most often, the skin belonged to Nina.

(Александр Чернобровкин, Чижик — пыжик, 2008)