Your fly is undone! 你的裤子前裆没拉上！

abc

fly作为名词的用法：
fly  (pl. flies)
INSECT 昆虫
1. [C] a small flying insect with two wings. There are many different types of fly.
   蝇；苍蝇:
   A fly was buzzing against the window.
   一只苍蝇嗡嗡地飞着,直撞窗子。
   Flies rose in thick black swarms.
   苍蝇黑压压地成群飞起。
IN FISHING 钓鱼
2. [C] a fly or sth made to look like a fly, that is put on a hook and used as bait to catch fish
   (作钓饵的)苍蝇,假蝇:
   fly-fishing
   用假蝇作饵钓鱼
ON TROUSERS / PANTS 裤子
3. [sing.] (BrE also flies [pl.])  an opening down the front of a pair of trousers / pants that fastens with a zip or buttons and is usually covered over by a strip of material
   (裤子的)前裆开口:
   Your fly is undone!
   你的裤子前裆没拉上！
   Your flies are undone!
   你的裤子前裆开着呢！
ON TENT 帐篷
4. [C] a piece of material that covers the entrance to a tent
   门帘
⇨ see also flies
▪ die / fall / drop like 'flies    (informal)  to die or fall down in very large numbers
   大批死亡；大批倒下:
   People were dropping like flies in the intense heat.
   酷暑中人们成批死去。
▪ a / the fly in the 'ointment
   a person or thing that spoils a situation or an occasion that is fine in all other ways
   扫兴的人；煞风景的事物
▪ a fly on the 'wall
   a person who watches others without being noticed
   不为人觉察的观察者:
   I'd love to be a fly on the wall when he tells her the news.
   他把这消息告诉她时,我想悄悄在旁观看。
   fly-on-the-wall documentaries (= in which people are filmed going about their normal lives as if the camera were not there)
   纪实影片
▪ (there are) no flies on 'sb    (informal)  the person mentioned is clever and not easily tricked
   某人精明得不会上当
▪ not harm / hurt a 'fly
   to be kind and gentle and unwilling to cause unhappiness
   连一只苍蝇都不肯伤害；心地善良
▪ on the 'fly    (informal)  if you do sth on the fly, you do it quickly while sth else is happening, and without thinking about it very much
   赶紧地；匆忙中


此外fly还可以作为形容词，要格外注意
adjective
(informal)
1. (BrE) clever and showing good judgement about people, especially so that you can get an advantage for yourself
   机灵的；机警的；不会上当的
2. (NAmE) fashionable and attractive
   时髦迷人的；漂亮的