Slack pack: yup, I meant the definition that aimless gave.

In fact, my recollection is that it's been used to mean a couple of things over time. A perhaps older (?) use of the term is to mean an unhurried and non-goal-oriented manner of hiking. But in thru-hiker circles, I've only heard it to mean "someone else is hauling much of your stuff so you can just walk with a day pack".