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 overweight the person the more it manifests visually as sucking up the stomach.
Suck it up and re-shoulder that pack.
Either Private Gump sucks it up or the platoon'll gimme twenty more pushups.
Perhaps the most comfortable of all combos, the Adidas Sandals and Socks Combo provides unrivalled comfort along with looks that will pull the most hard-to-get bitches. Common amongst Europeans (mainly Croatians/Bosnians), such a combo dates back centuries and has proven to withstand the tests of time to evolve from leather to the new plastic, Adidas sandals.
Slipping into his Adidas Sandals and Socks Combo, Dave was ready to tackle treacherous endeavours and whatever challenges life throws up, as only such a combo can handle impossible tasks whilst providing extreme comfort, style and sex appeal.