"Galactic cannibalism" refers to a phenomenon in space where larger galaxies consume or merge with smaller galaxies. It's called cannibalism because it's similar to how some animals eat others of their own kind. Full definition
Some stars in the spheroid are the remains of galactic cannibalism, having come from dwarf galaxies that fell into the spiral galaxy, were ripped apart by powerful tidal forces, and were incorporated into the larger galaxy's structure. (discovermagazine.com)
Bullock notes that this is not the first time such evidence has been found — in fact, some of McConnachie's co-authors published evidence for galactic cannibalism in Andromeda in 2001. (scientificamerican.com)