From under- +‎ brush.

• (General American) IPA^(key): /ˈʌndɚˌbɹʌʃ/

• Audio (US):

  (file)

underbrush (usually uncountable, plural underbrushes)

1. The small trees and other plants that clutter the floor of a forest.
   • 1892, Stanley Waterloo, A Man and a Woman:

     How quick the eye and hand to catch him [the ruffed grouse] when he rises from the underbrush and is out of sight in the wood before the untrained sportsman stops him with what is little more than a snapshot, so instantaneously must all be done!

• underbush
• undergrowth
• understory

underbrush (third-person singular simple present underbrushes, present participle underbrushing, simple past and past participle underbrushed)

1. (transitive) To clear (an area) of underbrush.
2. (intransitive) To work among the underbrush.

• undershrub