From Wikipedia
The term spiritual is derived from spiritual song. The King James Bible‘s translation of Ephesians 5:19 says: “Speaking to yourselves in psalms and hymns and spiritual songs, singing and making melody in your heart to the Lord.” The term “spiritual song” was often used in the black and white Christian community through the 19th century (and indeed much earlier), and “spiritual” was used as a noun to mean, according to the context, “spiritual person” or “spiritual thing”, but not specifically with regard to song. Negro spiritual first appears in print in the 1860s,[7] where slaves are described as using spirituals for religious songs sung sitting or standing in place, and spiritual shouts for more dance-like music.
Some scholarship claims that songs such as “Wade in the Water” contained explicit instructions to fugitive slaves on how to avoid capture, and on which routes to take to successfully make their way to freedom.[11] “Wade in the Water” allegedly recommends leaving dry land and taking to the water as a strategy to throw pursuing bloodhounds off one’s trail. “The Gospel Train“, “Song of the Free“, and “Swing Low, Sweet Chariot” are likewise supposed to contain veiled references to the Underground Railroad, and many sources assert that “Follow the Drinking Gourd” contained a coded map to the Underground Railroad.
Lighten up, Brighten up 