Just some suggestions for increasing the working vocabulary of the githyanki language. I don't have a good sense of the Githyanki sentence structure, so I'm not sure if I got the compound words right.
gath or gaath or gathisk: thrall/slave. Gathisk is only used in the specific case of psionic or magical enthrallment (as opposed to 'physical' slavery). Occasionally any of these words may be used, very insultingly, to describe ideological renegades.
ekor: master (specifically, one who keeps thralls). This word is ALWAYS negative/derogatory in the Githyanki language.
ula: brain. This word seems to be clearly derived from the Undercommon uliu or sometimes ulius, referring to the physical brain. A number of githyanki words referring specifically to the brain or skull can be traced to Undercommon linguistic roots.
ul: (uncommon) suffix for matters pertaining specifically to the physical brain. Most often seen in medical terminology, psionic terminology, or references to brain-eating.
isk: suffix (derived from "isk) used in multiple compound words involving the mind.
yarath: hunger. (The particular hunger of illithids is yarath'ul, i.e., 'brain hunger.')
ik'tha: dagger
ral: "strong" or "powerful"
ral'a: strength/power
ral'a'isk: strength of will; resolve; willpower. (A person with great ral'a'isk shrugs off mind blasts and makes it look easy.)
ralisk: a psion (literally, 'powerful mind')
zath: meditation or contemplation
koshaath: 'great' or 'mighty'
tel'kul: (verb) devour. (Always negative or derogatory in Githyanki. Particularly used in relation to illithid-brain eating, but may be used to describe (other) monsters eating, or a particularly gluttonous person's eating habits.)
k'ya (archaic form: kiya): A word that can mean 'creature,' 'monster,' or 'beast,' depending on circumstances. Something that's not clearly an illithid, one of the gith races, a graith humanoid, or an undead. Not applied to animals that githyanki would consider 'normal.' Sometimes translated as 'thing,' but the word never describes an object, only a living creature. A k'ya might be a green slime or a displacer beast.
za'vesh: extremely foul; obscene; unclean. (The word in Githyanki does not usually have sexual connotations; 'obscene' is more likely to pertain to concepts like brain-eating and enslavement, though it occasionally can refer to 'intimate relations' with githzerai.)
ekor-k'ya: 'master-thing,' a deity
gath-i ekor-k'ya: 'god-slave', a priest
za'vesh ekor-k'ya: 'obscene master-thing,' a reference to Ilsensine