Fetching details from the dictionary...

tenacious; tough; enduring; dogged; headstrong; stubborn; obstinate; unyielding
彼はしぶとく諦めずに交渉を続けた。
He tenaciously continued negotiating without giving up.
しぶとい雑草はなかなか根絶できない。
Tenacious weeds are hard to eradicate.
しぶとい風邪で一週間も寝込んでしまった。
I was bedridden for a week with a stubborn cold.