Terry Callier was an American jazz, soul and folk guitarist singer, song-writter from Illinois. Spin comes from his album The New Sound of Terry Callier, recorded in 1964.
Anne-Marie from Redland Voices (3rd chapter of Gurt Lush Choir) gave this information about the origin of the song:
"According to my pal from American South, the spin here refers to a wheel spinning cotton and given that the slaves had their small children and/or babies at work with them (?) my guess is it was a lullaby too.
It fits with all these songs being fundamentally work songs.
Patti (my US friend) was quite tearful as she remembers her family "housekeeper" singing it while she worked...