掉以轻心 (diào yǐ qīng xīn) — to let one's guard down, to be careless (often in negative constructions)

Definition

Examples

• 这件事很重要，我们不能掉以轻心。
  Zhè jiàn shì hěn zhòng yào, wǒ men bù néng diào yǐ qīng xīn.
  This matter is very important; we can't take it lightly.
• 虽然已经准备很久了，但比赛时还是不能掉以轻心。
  Suī rán yǐ jīng zhǔn bèi hěn jiǔ le, dàn bǐ sài shí hái shì bù néng diào yǐ qīng xīn.
  Although we've been preparing for a long time, we still can't let our guard down during the competition.
• 他对这个问题掉以轻心，结果出了大错。
  Tā duì zhè ge wèn tí diào yǐ qīng xīn, jié guǒ chū le dà cuò.
  He was careless about this issue, and as a result made a big mistake.