1. What does it mean?
패작 describes intentionally producing losses rather than trying to win. In ranked games it can involve lowering an account's standing or otherwise manipulating competitive results. It is different from simply having a bad game.
Also written or said as paejak
2. The nuance
This is a serious accusation about intent. Mistakes or a losing streak are not sufficient evidence, and deliberate match disruption may violate the game's rules.
3. Examples
“연패했다고 바로 패작이라고 단정하지 마.”
Don't assume someone is throwing matches just because they're on a losing streak.
“운영 공지에서 고의 패작을 금지한다고 했어.”
The operator's notice said intentionally throwing matches is prohibited.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.