Entry 1 sense Webster, 1913 Underbrush /ŭn'-dẽrb-rŭshˌ/ · Un·der·brush · IPA /ˈʌndɚˌbɹʌʃ/ 01 n. Shrubs, small trees, and the like, in a wood or forest, growing beneath large trees; undergrowth. 1. Shrubs, small trees, and the like, in a wood or forest, growing beneath large trees; undergrowth.