I may have solved the mystery of why coffee grounds repel slugs. Maybe.
I put down some coffee grounds around my broccoli plants. We had a spell of sunny and warmish (for end of Autumn May) weather. Last night we had a few mm of rain. Today I have found several dehydrated slugs on the coffee grounds. My deduction is that the coffee grounds dehydrate the slugs as they crawl across it much as cigarette ash does. If so, I protect my plants AND add some organic mater & fertiliser to the soil. Perfect.
rob