Морочить голову (to pull somebody's leg). Морочить means to fool. So when you “Морочишь голову кому-либо” you just try to cheat or to joke on somebody by telling him a lie. If you don't believe what a person is telling you, then you can tell him: “Не морочь мне голову!” (don't you try to pull my leg!)
It is believed that the origin of the verb Морочить/Заморочить (to fool) is related to the Old Church Slavonic word мрак which means darkness, отсутствие света (absence of light) and in a figurative sense неизвестность (unknown).
In modern Russian, you can find some words related to мрак: for example the word морока (confusion), a nonsense, incomprehensible and tedious action or обморок (fainting).
It recently gave way to a new slang expression “Не заморачивайся!” which means “Не принимай это в серьёз!” (don't take this seriously!)
Тамара Мелентьева
