hang on/hold on like grim death

hang on/hold on (to somebody/something) like grim ˈdeath

hold somebody/something very tightly, usually because you are afraid or determined not to let go: As the horse galloped off, you could see poor Sarah hanging on like grim death.The robbers tried to steal my bag, but I held on to it like grim death.
The word grim (= very serious or unpleasant) is often used to describe death.
See also: death, grim, hang, hold, like, on