proof that someone who is thought to have committed a crime could not have done it, especially the fact or statement that they were in another place at the time it happened

He has a cast-iron (= very strong) alibi - he was in hospital the week of the murder. 他有不在犯罪现场的铁证——谋杀发生的那一周他在住院。

an excuse for something bad or for a failure

After eight years in power, the government can no longer use the previous government's policy as an alibi for its own failure. 本届政府已经执政8年，面对自己的失误，它再不能拿前任政府的政策作挡箭牌了。