This derogatory shop term refers to budget-brand or heavily worn tires that offer virtually no traction, especially in the rain. The name implies that driving on them will inevitably cause the vehicle to slide off the road and into a ditch. Mechanics use it to warn customers about the severe safety risks of prioritizing price over tread compound quality.
Heard on the job
I wouldn't push it too hard in the rain, mate, you've got a set of absolute ditchfinders on the rear axle.
Insider tip
If a customer insists on keeping their ditchfinders, make sure to document their refusal of new tires on the repair order to protect the shop from liability.