Late Middle English (in an earlier sense). Apparently from dog + bolt, although this may not be the case in senses A. 1 and A. 2. In spite of the chronology, it is possible that sense A. 2 was the earliest meaning, and that the word is from dog + bolt, in which case dog perhaps carries the connotation ‘of little value’, perhaps in this context specifically ‘such as might be shot at a dog’; sense A. 1 may (in spite of the chronology) show an extended use of this.