Based on the search results, "deco" in text message can refer to two different things:

1. Text Decoration: In CSS, "text-decoration" is a shorthand property for "text-decoration-line", "text-decoration-color", "text-decoration-style", and "text-decoration-thickness" properties. It is used to add visual effects to text, such as underlining, overlining, or striking through text.

2. Deco Mail: "Deco Mail" or "Decome" is an email feature available on Japanese mobile phone services that allows users to use an online rich-text editor so that they can include decorative images (such as emoji), animations, and backgrounds in their email messages, which are encoded as HTML emails. It is a part of Japanese mobile phone culture and was widely used in the 2000s, but has largely declined from the widespread adoption of smartphones and messaging apps.

Therefore, the meaning of "deco" in text message depends on the context and can refer to either text decoration or Deco Mail.