His constant glances at his watch were a clear giveaway that he was in a hurry.

giveaway 是由动词短语 give away（泄露、赠送）组合而成的名词，字面意思是"泄露的东西"。在这里指"无意中暴露真实情况的行为或迹象"，即"暴露、泄露"的意思。