1. Experience or cause to experience hallucinations, paranoia, or other frightening feelings as a result of taking a mind-altering drug. For example, They were freaking out on LSD or some other drug. [Slang; mid-1960s]
2. Behave or cause to behave irrationally and uncontrollably, with enthusiasm, excitement, fear, or madness. For example, The band's wild playing made the audience freak out, or It was such a close accident, it really freaked me out, or She freaked out and ended up in the psychiatric ward.
[Slang; 1960s] Also see flip one's lid; wig out.