Noah Webster's Dictionary (n.) That which grows under trees; specifically, shrubs or small trees growing among large trees.

Multi-Version Concordance Undergrowth (2 Occurrences) Job 30:7 Among the bushes they bray; under the nettles they are gathered together. (See NIV) Isaiah 17:9 In that day your towns will be like the waste places of the Hivites and the Amorites which the children of Israel took for a heritage, and they will come to destruction. (See NIV)