A racial slur often targeted at members of the betailed Saiyan race. Saiyans find this offensive because it suggests their uncivilized nature, which non saiyans are expected to ignore. It should also be noted that, Saiyans do customarily refer to each other as Monkey, which when used in such a context, is a term of endearment.
"I can't believe I was defeated by a MONKEY!"
"Hey man, you hear that Monkey Kakarot just pwned Frieza?"