A gag is a humorous trick that you play on someone. 恶作剧 [美国英语] [非正式]
例：
Richard must have thought colleagues were playing a gag on him.
理查德一定以为同事们在对他搞恶作剧。
[T] ~ sbto put a piece of cloth in or over sb’s mouth to prevent them from speaking or shouting 捂住，塞住（某人的嘴）
The hostages were bound and gagged.
人质被绑起来并被人用东西塞住了嘴。
[T] ~ sb/sthto prevent sb from speaking freely or expressing their opinion 压制…的言论自由；使缄默
The new laws are seen as an attempt to gag the press.
人们认为新法律企图压制新闻界的言论自由。
see also gagging order
[I] ~ (on sth)to have the unpleasant feeling in your mouth and stomach as if you are going to vomit 作呕 SYN retch
She gagged on the blood that filled her mouth.
她因嘴里充满了血而作呕。