Best definition
verb: when one pulls the back of a chair, couch, or loveseat to make the person sitting in the chair

fall back backwards and scream like a little girl. Originated in Washington, DC.

Joe: What did you guys do last night?

Erin: Nothing. We chilled in the lounge until Fred came and Walstamed me. I fell on my ass and hit my head.

Walstam: define #2
A Walstam is a mean, giant Swede who likes to tickle people and flip over the chairs they are sitting in when they are least expecting it. Walstams make rude and inappropriate jokes. Sometimes they’re kind of creepy.
Don’t worry about it, he’s a Walstam. It’s what he does