I looked this up once to determine if it was real or just hilarity. And the answer is great. From [Sims Fandom](https://sims.fandom.com/wiki/Reticulating_splines) site:

> The words 'reticulate' and 'spline' both have dictionary definitions, which has led several people to determine a meaning for the phrase, such as "to make a network of curved elements." However, Will Wright stated in an interview that the term itself is meaningless, as SimCity 2000 does not reticulate splines when generating terrain; the phrase was included in the game because it "sounded cool."