being or containing an acid; of a solution having an excess of hydrogen atoms (having a pH of less than 7)
"The lemon juice tested as highly acidic when we dropped the litmus paper into it."
Having a pH less than 7, or being sour, or having the strength to neutralize alkalis, or turning a litmus paper red.
"The lemon juice tasted so acidic that it made my lips sting immediately."
The word acidic comes from the noun acid combined with the suffix -ic. It is a doublet of the variant spelling acidy.