APAD: Queer someone's pitch

"to queer someone's pitch" means to interfere with someone’s plans or spoil their chances of success.

It is an informal British expression often used when someone deliberately makes it difficult for another person to achieve what they want.

The "Pitch": In the 19th century, a "pitch" referred to the specific spot where a street performer (like an acrobat or musician) or a market trader set up their business.
"Queer": In this context, it is an older British slang verb meaning to spoil, ruin, or sabotage--perhaps by a rival setting up nearby or someone causing a disturbance—thereby ruining the trader's opportunity to make money.

If you "queered" a performer's pitch, you did something to drive away their audience or make it impossible for them to work, such as setting up a noisier act nearby.

In modern usage, it is simply a way to say someone has disrupted or sabotaged a situation, often unexpectedly.