grim

like (or for) grim death with intense determination.

☞ The variant for grim death, which makes no literal sense, probably arose by contamination with for dear life (seelife).

2014Daily Telegraph With Ségolène spitting nails, and Valérie clinging on to her man for grim death, Hollande finally landed the top job in May 2012.

the Grim Reaper a personification of death in the form of a cloaked skeleton wielding a large scythe.