To make a very idiotic, or "jackass" assumption. Concatenation of the words "jackass" and "assume"
GF: Don't think you can hide yourself from me. I know that you're checking out that girl who you think is better looking than I am.
Me: Just because I'm looking at that girl doesn't automatically mean I'm checking her out. Maybe I just want to see if she's one of my classmates at my previous school. Maybe I'm not looking at her, but rather, the traffic that's behind her. Don't jackassume things.