What is "undergrow"

Undergrow \Un`der*grow"\, v. i. To grow to an inferior, or less than the usual, size or height.
--Wyclif.

Undergrow \Un`der*grow"\, a. Undergrown. [Obs.]
--Chaucer.

vb. 1 (context intransitive English) To grow to an inferior, or less than the usual, size or height. 2 (context transitive English) To grow underneath.

v. grow below something; "The moss undergrew the stone patio"