たかをくくる

expression, godan verb (-ru)

underestimate; make light of; think little of

Used when someone misjudges a person, situation, or task as being easier or less significant than it really is, often leading to trouble. The phrase literally means 'to tie up the amount', implying a casual, dismissive assessment.

• 相あい手てを高たかをくくっていたら、試し合あいに負まけてしまった。

  I underestimated my opponent and ended up losing the match.

• この仕し事ごとを高たかをくくって引ひき受うけたが、思おもったより大たい変へんだ。

  I took on this job thinking it would be easy, but it's harder than I expected.