Cows have only been domesticated in the last 8000 years. Before that, they were running around as mad as lorries. The human digestive system hasn't got used to any dairy products yet.
12 seconds sound clip from the Snatch movie soundboard.
You can hear this line at 01:10:24 in the Blu-ray version of the movie.
Quote context
[...]
- Well, you wanted to know whether or not they worked.
- I didn't mean try it in the car, Sol, you arsehole!
- What are you gonna do with that, pick his teeth?
- Wipe the butter off and put it away.
- Look, there's a proper blade back there.
- Well, fuck me, Tommy. What have you been reading?
- Me? Oh, no, not me. I wouldn't know what to do with it.
[...]