They ended up gatecrashing the birthday party without an invitation, which was really awkward.

### 解释 gatecrashing 是动词 gatecrash 的现在分词形式。gatecrash 由 gate（大门）和 crash（碰撞、闯入）组成，字面意思是"撞门而入"，引申为"未经邀请擅自闯入（聚会等场合）"。