Be joined at the hip:
(Of two people) be inseparable.
“Be joined at the hip（臀部连在一起）” 实际所表达的意思是 “某人和自己的一个朋友总是形影不离，密不可分”，因此人们经常看到这两个人在一起。它也暗示若见到其中一个人单独行动是很反常的。
例句
My best friend from high school and I did everything together – we were joined at the hip.
我上高中时最好的朋友和我曾经做什么事情都一起，那时我们俩形影不离。
Where is Tom? You two are normally joined at the hip.
汤姆在哪儿？你们俩通常是形影不离的。
Whenever you see Sarah, you know Lucy isn’t far away – they’re joined at the hip, those two!
你一看到莎拉，就知道露西肯定在不远的地方，她们俩真是密不可分！
