I'd generally say that when people talk about the liveliness of a system they're talking about the high frequency response. You get more zing off your cymbals and more shine off your guitar strings when you have a more tipped up system in the top end. It is not nonsense... it's a matter of trying to describe the effect of the sounds you're hearing. If someone says a system sounds lively, and you roll off the top at 15khz and beyond, they'll probably tell you it sounds less lively. It is the perception of the sound, which is hardly nonsense, just describing the perception in how you actually understand the sound you are hearing. And yes there is some imprecision in that sometimes people mean slightly different things by the terms they use, but perception is inherently subjective so it's bound to happen.
Please try to be a bit less dismissive and rabid.