Blowby is basically the leakage of compressed air-fuel mixture and burned gases past the piston rings into the crankase.

窜气，基本上是被压缩的空燃混合气和燃烧后的气体的泄漏，它通过活塞环进入曲轴箱。

youdao

In fact, there had been evidence of leakage, what the engineers called "blowby, " on an earlier shuttle flight.

NPR: Challenger: Reporting a Disaster's Cold, Hard Facts

The engineers had resisted the launch, had recommended against it, citing the "blowby" in an earlier low-temperature launch and studies of the elasticity of the o-rings.

NPR: Challenger: Reporting a Disaster's Cold, Hard Facts