His muscles bunched under his shirt.

他衬衫下面的肌肉紧绷绷的。

《牛津词典》

When they frisked him at the airport, they found a knife hidden under his shirt.

他们在机场对他进行搜身检查时，发现他的衬衣里藏着一把刀。

youdao

The Spanish captain saw him take his hand from the knife hidden under his shirt.

西班牙船长看到他的手从藏在衬衣下边的刀子上松开了。

youdao

He had not been hurt. He was wearing a steel plate under his shirt.

VOA: special.2010.03.17

The white silk shirt and the blue and lilac tie felt cool and smooth under his fingers.

VOA: special.2009.04.11

He saw the puzzled look on my face, reached under his shirt and took out a gun.

BBC: Hometown Karachi: Returning to report on a bleeding city

Just before the door closed, though, he pulled some papers from under his shirt and handed me the report.

CNN: The Ghosts of Shanghai

So Vinny, the high school graduate from New Jersey with hair coming up from under his shirt, now had to run his bond trades next to a PhD from MIT who could use mathematical models to understand the option value of the security.

FORBES: Michael Lewis and the Cultural Transformation of the Media Business