It is a unit of density. Density is defined as mass per volume.
A slug is an English unit of mass.
It is defined as a mass that accelerates 1 ft per sec per sec (ft/sec2) when a force of one 1 pound is applied to it.
It is equivalent to 14.5939 kilograms.
ft3 is an abbreviation for cubic feet, which is an English unit of volume.