自演乙 originally means "We appreciates your sockpuppet playing" in Japanese.

Overview

In Japanese culture, praising yourself is considered narcissistic and often frowned upon. Some of the tags, therefore, should be tagged by other users, not the creator himself. Tapping such tags by the creator is considered sockpuppeting or attention-seeking.

For example, Original 100 Users Bookmark at an original illustration can only be tagged when the illustration was 100 Users Bookmark which can be checked from the number of hearts.

It may also caused by misunderstanding the tag's meaning. A classic example is Incredibly Cute (なにこれかわいい): it means "The illustration is incredibly Cute", not "The character of the illustration is incredibly Cute", but some creators may misunderstand the tag.