The term 'turn one's back on' is an idiom that means to reject or IGNORE someone or something. It can be broken down into three crossword solutions: IGNORE (6 letters), SHUN (4 letters), and SPURN (5 letters). To IGNORE someone or something is to pay no attention to it, while to SHUN is to deliberately avoid contact with them. Finally, to SPURN is to reject or show contempt for someone or something. All three words are synonyms for the phrase 'turn one's back on'.