A phrase stated after something/someone has been easily fooled or lured in. It has its roots in fishing lingo, referring to a fish having fallen for the bait easily; swallowing the hook, the fishing line, and the sinker along with it.

When someone goes to the gym and spends 90% of the time on their phones scrolling through social media

Hook, Line and Sinker is an act of severe punishment for someone who has wronged you, or acted like a bitch in general. One or two people with restrain or "hook" the asshole. If you don't know what hook means, watch The Gangs of New York. You place index/middle fingers on the inside of someones cheek and force the mouth open, in agonizing pain. The line= pee and the sinker= poo.