Mon-keigh just functions as a sort of slur for non-eldar, most commonly used for humans in-universe, there's not really much further subtext. It is similar to the actual human word monkey to invoke a sense of arrogance, but it's changed cuz it's just not exactly kosher to refer to people as monkeys irl (even though scientifically speaking, humans are monkeys).

Apart from that, it's mostly just a reference to this