This is the 5639th most frequent Japanese word.
しつこい
Persistent; insistent; overly tenacious.
In this sentence, 'しつこい' is used to describe a person's persistent or insistent behavior in an action.
彼はしつこく質問を繰り返した。
He persistently repeated questions.
Here, 'しつこい' describes the stubborn or difficult-to-remove nature of the stain.
しつこい油汚れが落ちない。
The sticky oil stain won't come off.
In this context, 'しつこい' characterizes an unwelcome insistent nature of the solicitation.