Someone who has absolutely no willpower and will follow the anyone into any kind of debauchery.
Guy 1: Let's go out drinking
Guy 2: Nah mate, I've just recovered from a 20 hour bender
Guy 1: Ah, go on then...
Guy 2: Ah, OK, why not
Guy 1: Mate, you've got the will power of a lemming
While the 'Will to Live' is simply life's instinct of self-preservation, 'Will to Power' is the idea that living creatures do not simply want to survive, but seek power to grow, spread, expand strength, and become dominant.