L.U.P. (I shold have capitalized it) stands for Lead Units of Pressure. The old standard way of measuring pressure of shotshells was to use a special barrel set up to insert a precisely shaped lead "crusher" between a piston that went into the chamber, and the fixture on the outside of the barrel. The lab would measure the length of the crusher before inserting it in the piston. Then they would tighten the fixture over the crusher. After firing the shell, they'd take out the crusher and measure the length again, and compare the lengths in their chart to get the pressure. It's not exactly like pounds per square inch (PSI) and they don't compare directly. Higher-pressure ammo (Rifles, and some handguns) used to use a copper crusher instead of lead, and you would find published data on C.U.P. for Copper Units of Pressure.
I think all the ammo makers test pressure with electronic gear now, but as I mentioned above, they don't publish pressure any more. Folks that offer reloading data do publish pressures, though. Hodgdon lists it as "PSI", and Aliant shows "Approx. PSI" in their tables.