To prepare one's self to put forth greater effort in overcoming an obstacle (of any type; physical, mental, etc.)
origin - (probably military drill sergeants) preparing for any strenuous physical task invariably requires fully engaging the stomach muscles and the more overweightthe person the more it manifests visually as sucking up the stomach.
Suck itup and re-shoulder that pack.
Either Private Gump sucks it up or the platoon'll gimme twenty more pushups.
A harsh term friends say to their friends so that they can feel superior and get their fragile friends to do whatever they're told. This is extremely annoying and hurts people deep down inside so it shouldn't be used unless you know it doesnt hurt them.
girl: Hey
Evil: why are you crying?
Girl: My life is falling apart, my boyfriend dumped me, my grades are falling and my parents hate me.
Evil: Stop being emo and suck it up, jeesh, you're depressing me.
Girl: (sniffle) sorry....