From Brotato Wiki
Peacock is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Peacock grants a consistant +25% exp gain, but also a temporary +100% exp gain for the next wave. However, enemies for the next wave also have +50% damage during that wave.
Timing your purchase of the Peacock is key. Buying the peacock before a wave that grants a large amount of experience, such as wave 9 or a horde wave, can give you a very large boost.
However, be careful about buying the peacock before a dangerous wave, such as wave 14 where the large amount of projectiles can be especially dangerous, or Elite waves.
You can lock the Peacock in the shop for a round or more to make the timing better, but remember that locking an item means seeing less items in the shop, so don't lock it for several rounds.