追本窮源

see 追本溯源[zhui1 ben3 su4 yuan2]

root / stem / origin / source / this / the current / original / inherent / originally / classifier for books, periodicals, files etc

to sculpt / to carve / musical instrument (old)

to chase / to pursue / to look into / to investigate / to reminisce / to recall / to court (one's beloved) / to binge-watch (a TV drama) / retroactively / posthumously

root / source / origin

poor / destitute / to use up / to exhaust / thoroughly / extremely / (coll.) persistently and pointlessly