The Collaborative International Dictionary of English v.0.48:
backbench \back"bench\ (b[a^]k"b[e^]nch), n. 1. any of the seats occupied by backbenchers in the House of Commons of Great Britain. [WordNet 1.5]WordNet (r) 3.0 (2006):
backbench n 1: any of the seats occupied by backbenchers in the House of Commons