GP is a nice reverse engineered explanation, but the actual origin of using seconds as a unit is to avoid having to compare metric and imperial units of speed. Isp is just exit velocity divided by g and it's conveniently in seconds, a pretty universal unit. And "one pound of thrust" only makes sense when you're in Earth's gravity field with gravity losses. Otherwise it's more precise to use Newtons for thrust.
> conveniently in seconds, a pretty universal unit.
Thankfully! Imagine if systems/cultures commonly differed in fundamental measures of time, just as they sometimes do for volume, distance, weight, etc.
