The phrase "lose cool" means to become upset, angry or frustrated. There are several synonyms that can be used in place of this phrase. One term that is often used is "lose one's temper". This means that someone becomes angry and starts to behave in an uncontrolled manner. Another synonym is "fly off the handle". This phrase is similar to "lose one's temper", meaning that someone becomes angry suddenly and without warning. Other terms include "lose control", "blow a fuse" and "go ballistic". All of these terms describe the same concept - losing one's cool and becoming angry or upset.