1. What does it mean?
역캐리 reverses 캐리. Players use it when a costly mistake or poor performance plays a large role in their own team's defeat, joking that the person helped the opponents win instead.
Also written or said as yeokkaeri
2. The nuance
It can be self-deprecating, but aimed at a teammate it is harsh blame. It does not prove intentional sabotage; ordinary mistakes and deliberate 패작 are different.
3. Examples
“마지막 실수는 내가 역캐리한 셈이네, 미안해.”
That last mistake basically handed them the win; sorry.
“한 번 실수했다고 계속 역캐리라고 놀리지는 마.”
Don't keep mocking someone as the reason we lost over one mistake.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.