Something brittle is easily broken. Do you have brittle bones? Then no football or rugby for you.
Besides meaning easily fractured and emotionally cold, brittle is also a type of candy made of cooled sugar. To make peanut brittle, bake the ingredients on a cookie sheet then let the brittle cool into sheets which you break into pieces (and then eat — wreaking havoc with your braces). It's ok to use the adjective brittle freely to describe lots of things that will break easily — such as a heart, a theory, or a poorly equipped army.