'Shook' means shocked, stunned or shaken up. 'I was shook' = 'I was stunned'. Originally African-American Vernacular English (used in 1990s hip-hop to mean scared or rattled), mainstreamed via internet culture in the 2010s. If your teen says they're shook, they are reacting strongly to something — could be good or bad news. Harmless conversational vocabulary.