bundle做名词是“束；捆”，把自己穿的像被捆起来，就是很暖和的意思啦。

bundle sth up就是“把…捆扎起来”，而bundle (sb) up才是“（使…）穿得暖和”。

The kids were bundled up in coats and scarves.

孩子们穿着大衣，系着围巾，裹得严严实实。