Check a thesaurus. Stacking and packing are symantically related and that's exactly what we're dealing with regards to the GOP's current argument.
They changed the rules in 2016 to prevent Merrick Garland being appointed to the court's vacancy (after Scalia's death) by refusing to hold senate hearings or even a vote on his nomination claiming that because it was an election year, the court should continue with 8 justices till after the election (9 months away) wherein the next administration would fill the position.
In 2020, the GOP changed the rules again, completely reversing their prior position that no new justice should be appointed in an election year and swiftly installed Amy Comey Barrett to the Court within weeks of Ruth Bader Ginrich's death and did so while the federal election was already under way.
Unilaterally changing the rules as they did to gain an upper hand in the courts is what is known as "stacking".