You could tell that they were itching to leave.

were 是 be 动词的过去式复数形式，表示"是"或"处于某种状态"。 itching 是动词 itch 的现在分词形式，itch 本意是"发痒"，在这里引申为"渴望、迫切想要"。 to 是不定式符号，后面接动词原形 leave，表示"离开"的动作。 were itching to 是一个固定搭配，字面意思是"处于渴望要...的状态"，引申为"迫不及待地想要..."。