Weakness is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:weakness
Give Weakness II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Weakness reduces your melee attack damage by 4 points per level, and its splash potion is the key ingredient for curing zombie villagers.
Brew a Potion of Weakness by adding a fermented spider eye directly to a water bottle, no awkward potion needed, which makes it the cheapest potion in the game. Gunpowder turns it splash for villager curing and PvP.
The curing ritual is splash Weakness plus a golden apple on a zombie villager, then wait for the conversion, which trades you permanently discounted villagers. In combat, splash Weakness blunts enemy melee, though most players reach for damage instead.
The effect ID for Weakness is minecraft:weakness. Java Edition 1.13+ /effect commands use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 18.
Run /effect give @p minecraft:weakness 60 1 to get Weakness II for 60 seconds. The first number is the duration in seconds; the second is the amplifier (0 = level I, 1 = level II). Remove it with /effect clear @p minecraft:weakness.
Weakness is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Hit the zombie villager with a Splash Potion of Weakness, then use a golden apple on it and wait a few minutes. The villager converts with heavily discounted trades, the foundation of trading halls.
Add a fermented spider eye straight into a water bottle at a brewing stand. It is the only potion that skips the nether wart step, so it costs almost nothing to make.