Pervert the course of justice
Definition 1 of 1
Pro Tip 1/3Legal Severity
The phrase indicates a serious offense within legal contexts, implying intention to disrupt justice. 
If you lie to protect a friend, you might pervert the course of justice.
Pro Tip 2/3Not Just Lying
Involves any act that hampers legal proceedings, not only giving false statements. 
Destroying documents can also be seen as an attempt to pervert the course of justice.
Pro Tip 3/3Context Specific
It's mostly used in formal, legal discussions and rarely in everyday conversation. 
In court, they discussed whether witness intimidation was perverting the course of justice.