An acidic thing or place has too much acid in it, which can make it hurt your skin or other living things.
When we talk about something being acidic, we usually mean that it's sour-tasting or has a strong smell. Like if you're drinking lemon juice and it makes your face pucker up - that's because lemons are very acidic. Or imagine going to the beach on a really hot day and feeling the sharp sting of saltwater in your eyes - the ocean is also quite acidic, especially when it's full of seaweed or other algae. Some people even have to worry about things like acidic rain, which can hurt plants and buildings.
Having a pH less than 7, or being sour, or having the strength to neutralize alkalis, or turning a litmus paper red.
