Bad Luck is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:bad_luck
Give Bad Luck II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Bad Luck does the opposite of Luck, worsening your odds on luck-based loot tables such as fishing treasure.
Like Luck, Bad Luck has no survival source in Java Edition and exists only for commands and custom content. It never appears in normal gameplay.
Map makers use Bad Luck to punish players with worse loot rolls. In vanilla play you will only ever see it referenced in effect lists like this one, so there is nothing to counter.
The effect ID for Bad Luck is minecraft:bad_luck. Java Edition 1.13+ /effect commands use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 27.
Run /effect give @p minecraft:bad_luck 60 1 to get Bad Luck 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:bad_luck.
Bad Luck is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Only through commands like /effect give @p minecraft:unlucky. No survival mechanic in Java Edition applies Bad Luck; it is a custom-content effect.
It subtracts from the luck attribute used by loot tables, slightly worsening results such as fishing treasure quality. In vanilla survival you will never encounter it.