Green’s Dictionary of Slang

1. (US black) to masturbate.

1968		J. Colebrook Cross of Lassitude 103: The oblique and penetrating language of the life. ‘Broad, give me some sky...’ ‘Nothing doing. I’ll go freak off with another Jasper. Both of us go for what we know...’.
1980		E. Folb Runnin’ Down Some Lines 238: freak off [...] 2. Masturbate.

2. (US black) to engage in unrestrained or unorthodox sexual activity.

a.1954		‘Ball of the Freaks’ in D. Wepman et al. Life (1976) 109: Grace was a sadist, one of the maddest, / Who’d freak off in the good Lord’s face.
1969		‘Iceberg Slim’ Pimp 63: You ain’t the only stud she freaks off with.
1972		Milner & Milner Black Players 156: Sometimes he would let her freak off with another woman.
1980		E. Folb Runnin’ Down Some Lines 162: Freak off mean indulge in some previous activity a little bit different from [long pause] ... You know, d’ usual sexual process.

3. to flirt, to attempt to pick up.

1969

‘Iceberg Slim’ Mama Black Widow 207: Charlotte is in the joint watching you freak off.

4. (US black) to express one’s enjoyment.

1967

L. Bruce Essential Lenny Bruce 67: Really freaked awf!

5. to decorate, esp. to create something beautiful.

1969		‘Iceberg Slim’ Pimp 111: She laid the bearskin and freaked the joint off with her lights and other crap.
1972		E. Folb Urban Black Argot 138: Freak Off Something to fix up something, particularly to equip one’s car with the latest accessories.
1980		E. Folb Runnin’ Down Some Lines 84: To freak off something means to fix up lavishly, decorate, or enhance an environment.

6. (US black) to have homosexual sex.

1980

E. Folb Runnin’ Down Some Lines 238: freak off […] 3. Engage in homosexual relations.