Githyanki are normally polite toward one another...but not always. None of the insults below should be used against a githyanki carelessly by the wise.
Venerator: Very insulting term for a priest. (The original meaning refers specifically to members of the illithid Venerator Creed.)
Battle-thrall: Someone who is excessively hot-headed and combative (but not too intelligent), even by githyanki standards. Sometimes used (by not-so-orthodox githyanki) to refer to the most fanatically religious of githyanki. Someone who is a little too quick to blindly follow orders. Someone who is a slave to his desire to fight, beyond all other considerations.
Brain-trader: A generic insult -- unless, of course, one actually is a brain trader. A serious insult to the honor of any githyanki so insulted. May imply that the person being insulted is either totally unscrupulous or an active servant of the illithid.
Brain-taster: A fancy term (translated directly from Undercommon) for an illithid performance eater. Another generic insult, but a little more likely than "brain-trader" above to be applied to a githyanki who's either regarded as really weird, or who is regarded as sadistic even by githyanki standards.
Breeding-thrall: One of the worst insults in the Githyanki language. Originally used to refer to thralls who were used primarily to make more thralls, and/or who were provided as rewards to other thralls for good behavior. May, however, be used to refer to someone of either gender. Sometimes used to refer to barbarian prostitutes or slaves, concubines/concubitors, and so on.
Personal-thrall: Illithids have a custom of each selecting one or more thralls to do things like take care of their personal quarters, be their personal bodyguards, dress them, carry their things, and so on: these are "personal thralls." When used, this insult usually implies that the person being insulted is excessively, slavishly devoted to someone or something, or actually serving a particular illithid.
Overseer: Illithid have another custom of using some slaves to act as overseers over the others. Implies that the person being insulted is extremely domineering and bullying.
Cattle: Generic insult. Another term for "illithid food."
Thrall-herder: Similar to "overseer" above, may be used to refer to someone with an extremely domineering personality. May be used more specifically to refer to (usually barbarian or illithid) members of the Thrallherd prestige class. Githyanki psi-dominators (and mage enchantment-school specialists) consider this term very offensive!
What's funny is, Githyanki are in fact slaves to their own arrogance and hatred.
Oo, I wanna see a much higher level character tell one that sometime.