slippery slope
noun [ S ] uk/ˌslɪp.ər.i ˈsləʊp/ us/ˌslɪp.ɚ.i ˈsloʊp/
滑坡效应（指坏情况或恶习一旦开始就很可能变得愈加严重）
a bad situation or habit that, after it has started, is likely to get very much worse
You're on a slippery slope once you start lying about your age! 你一旦开始对你的年龄撒谎，就再也停不住了！