Around min 10 into the movie, Jack Harmon tells Ethan not to chew the explosive gum by saying: "Hasta lasagna don't get any on ya" What does this mean? Why does he uses "hasta lasagna"?
Jack's basically warning him that's it's dangerous with a playful schoolyard phrase. Originally, this is supposed to be a play on Spanish goodbye phrases Hasta la vista, Hasta luego and Hasta manana ('Goodbye', 'till we meet again' and 'until tomorrow') with the childish twist of turning it into a food item and tacking a rhyme on in the end. It's similar to many other known childhood phrases, some of which are also about food and all of which are equally nonsensical, like:
"See you later, Alligator!"
"In a while, Crocodile!"
"What's shakin', Bacon?"
"Okey dokey, Artichokey!"
“Guess what? Chicken butt!”
So the 'lasagna' part is meaningless, it just sounds like a combination of La vista\Luego and Manana, or is simply a bastardization of one of them.
[Note that it's a double meaning, BTW: Jack tells Ethan it'll be the end of him (with the 'goodbye' part) and also to be careful around it ('don't get any on ya'). So he's being playful and instructive! ;)]