can go many ways, used in different instances, as "quit being a little bitch", or "do it, you pussy." and can also be used in place of "fuck you/off", or finally just to call someone afraid of something.
MAN1-"i don't want to go to worktommorrow."
MAN2-"boss up,lazy ass"
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"i guess we're just gonna have to boss up and go."
Take ownership of ones life, by directing the full capacity of their time, resources, and attention toward a specific direction, goal, or intent. In other words: step up, raise your standard, up your game, and take responsibility for the direction of your life and the full maturity of your dreams.