Reprehensible means deserving of blame or strong criticism. It is a strong word––your mother might forgive you for doing something bad, but something reprehensible? That's worse.
Near synonyms are blameworthy and culpable. Antonyms are blameless and praiseworthy. The adjective reprehensible is from Middle English, from Latin reprehensus, from reprehendere plus the suffix -ibilis "deserving of, capable of." If someone is reprehended, they are blamed or strongly criticized.