What is "in prison"

a. imprisoned, in a prison as a convict

In Prison is a 1915 song by composer Frederick Keel, composed in 1915 in the civilian Ruhleben internment camp on the former racecourse in Berlin where Keel was interned along with other artists and musicians including Edgar Bainton, Ernest MacMillan, Leigh Henry and Benjamin Dale. The text is from A Good Knight In Prison, or simply In Prison by William Morris from The Defence of Guenevere, and Other Poems 1858. The song In Prison, and the choice of text, is part of the musical testimony to Keel's activity as organizer and chair of the Ruhleben Music Society, consisting of 36 prisoners; the group's concerts were subsequently reported in the Times for 30 January 1919. Among other works of the Music Society Bainton also wrote his String Quartet in A major and MacMillan also a string quartet.