Literally: whole – body – whole – spirit
Alternately: Putting all of your physical and mental strength into something. “With all your soul and with all your might,” as they say. Doing one’s best; being utterly devoted to something.
Notes: A somewhat less dramatic version of the same idea is 全心全力 (zen shin zen ryoku), “whole heart, whole strength.”
At least one of my sources claims that this phrase comes from a 1919 novel titled 『或る女』 (Aru onna, A Certain Woman) by author Takeo Arishima (有島武郎).