Green’s Dictionary of Slang

1. (orig. US drugs) a drug-induced fantasy, reverie.

1968

N. von Hoffman We are the People Our Parents Warned Us Against 46: Some users [...] want to go on a ‘head trip’.

2. (US) something requiring challenging thought.

1971		Sun. Times 18 July 26: ‘Folie En Tête’ is her own title for this book, and exactly right. A head-trip, lasting 180,000 words.
1994		N. McCall Makes Me Wanna Holler (1995) 303: Such double-standards [...] worked a serious head-trip on me.

3. (US) deception or flattery.

1992

M. Leyner Et Tu, Babe (1993) 83: It’s the style, it’s the head trip I get into.