Another Polish saying "Tłumaczyć jak krowie na miedzy" that means "To explain [something to someone] like a cow in bounds".
This idiom is used to describe a person that is dumb above the average and/or has problems with understanding certain issue and you try to talk to him as plain as possible to explain it to him. Cow is believed to be a very stubborn animal and it is difficult to make it do something you want it to do - that's an etymology of an idiom.
In original Polish version word "bound" stands for a stripe of grass between fields (country land) that marks the real borders of a field so that a farmer would know where his field ends and start his neighbours.
- Wait, what was that bipolar junction transistor doing once again?
- Jesus, you're deaf or what? I've been talking to you like a cow in bounds for last 3 hours.