Jaffa term translating into two meanings: weakness, or death. Kek is not tolerated in Jaffa society; if one is weak, he may as well be dead. All Jaffa youth are born with the knowledge that kek is not accepted within the ranks.
"Kek" can also be used as the key word to commence an execution, rather than "fire."
Enemies - Apophis uses the word "kek" when he tells SG-1 that it is time for them to die.
Orpheus - Teal'c defines the term "kek" for SG-1, believing that the weakness of being without a symbiote has compromises his abilities as a warrior.