LORELAI: And then when we were older, scanning the neighbors’ houses for naked people [with the telescope].
CHRISTOPHER: [laughing] Found a couple of those.
LORELAI: [laughing] Mrs. Dominski undulating in her big fat Underalls is forever carved into my brain.
Underalls were a brand of underwear made by Hanes; it was a combination of underpants and pantyhose which was meant to eliminate panty lines, and marketed with slightly risque commercials. They were made from 1976 until the 1990s.
It seems to be a feature of scripts by Daniel Palladino that Lorelai makes nasty comments and jokes aimed at overweight people, especially women. It doesn’t really make any sense, since Lorelai’s best friends in Stars Hollow are all plus-sized women.
Are we meant to assume that Lorelai dislikes overweight women, but seeks them out as friends in order to make herself look better, or because they don’t threaten her fragile sense of self-esteem, while still making fun of them behind their backs? Because that isn’t a fun quirk, or a relatable human flaw – it’s revolting, and a sign of some kind of personality disorder.