しつこい Persistent, nagging. A person who accosts you in front of the train station and pesters you to join their cult or come to their stripper club. It’s like, “Who are you and why are you talking to me?” Unlike 執着、 しつこい is not limited to people:you can have a nagging injury or be irritated by endless pop-up ads.
(しゅうちゃく) Obsessive, anal-retentive, Linus and his security blanket. Someone who can’t go 4 minutes without biting their nails. Someone who is a borderline stalker. Unlike しつこい, someone who is 執着 is concerned with something they already have, and won’t give up, rather than something they want. Also, 執着 behavior goes on for weeks and months, whereas a しつこい usually applies to a single incident.