From Brotato Wiki
Handcuffs is a unique item. Only 1 of each unique item can be equipped, and it will then no longer be offered in the Shop, and no longer drop from loot crates.
Handcuffs provide a large amount of damage for a small cost. But it comes with the downside of locking your Max HP when you buy it. For example if you buy handcuffs while you have 50 Max HP, your Max HP can never be higher than 50.
However you can still buy items that lowers your Max HP and it can cause you to have less than 50 Max HP. The lock is only any upper bound, it doesn't prevent you from getting lower.
Handcuffs can be a powerful pickup late in the game if you've already gotten the Max HP you wanted for the wave 20 boss fight.