Three Manly Things:
1. Killing Something. Anything, be it an invading alien army, a dinosaur that a mad scientist brought back to life, a weed in your sidewalk, or a grunt in HALO, it doesn't matter. It garners respect from your enemies.
2. Watching TV in your underwear. Women would do this, but then men wouldn't be watching the TV, and then manly thing #2 would just be "in your underwear." Women don't watch TV in their underwear so that manly thing #2 can be "Watching TV in your underwear." Thank you ladies, now we can be manly.
3. Carrying heavy stuff. It helps with manly things #1 and #2.