* If you mean "shortening", the opposite could be expansion or elaboration. For example, the opposite of "USA" would be "United States of America".
* If you mean "short form", the opposite could be full form or long form. For example, the opposite of "Dr." would be "Doctor".
* If you mean "concise", the opposite could be detailed, verbose, or explicative. For example, the opposite of "He left" could be "He packed his belongings, kissed his family goodbye, and drove off in his car".
Ultimately, the best opposite for "abbreviation" depends on the specific context.