expression, godan verb (-su)
search high and low; search every nook and cranny
Idiomatic expression literally meaning 'to part the grass roots and search', used for a thorough, exhaustive search.
警察は草の根を分けて探しても犯人を見つけられなかった。
The police searched high and low but couldn't find the culprit.
The expression is a fixed idiomatic phrase; its literal image of parting grass roots to search conveys a thorough, exhaustive effort.