Green’s Dictionary of Slang

freak out v.

[freak v.3 (1)]
[1960s+] (orig. US drugs)

1. to experience an altered state of consciousness from the effects of a hallucinogenic drug; usu. an unpleasant effect; thus freaker-out n.

2. to worry someone, to disturb, to horrify – the level of trauma depends on context.

3. to engage in unorthodox or unrestrained sexual activity.

4. to go crazy, wild or out of control from fear or instability.

5. to experience intense emotional pleasure; thus freak someone out v., to engender such pleasure.

6. to be upset, worried; thus freaked out adj., worried, nervous, upset.

7. to back down, to retreat from a position.