Traditionally the Devil card of the tarot stands for excess, overindulgence and self-centredness. However the the colloquial implications of the idea add layers of ill-fated, even evil meanings to it. By and large, it is always a sign that someone around, the situation or your own emotion cannot be trusted in the moment.
Osho calls this tarot card conditioning, the 15th in the Major Arcana, and his inferences are based partially on the nature of glorifying darkness, such is as the way of the world.
The Devil symbolises our shadow side, the parts of ourselves that we prefer to deny or ignore. Being overtly concerned with appearances or social status, those that are not meaningful and that take our power away from our spiritual growth.
This card represents creative energy in its most material form. Creativity also in its dark side, for the ungoverned creative may be diabolical, such as the rule of order in chaos.
There are two distinct ways of seeing this card, with some seeing it as a warning to avoid temptation and others as an invitation to explore the hidden depths of the psyche. Either way the theme to watch out for here is overindulgence, and to it, extreme caution is recommended to your surrounding.
The Devil symbolises a focus on the material and meaningless, the illusion that nothing exists beyond that, a denial of the gifts and mysteries of spirit.
In your awareness is always the greatest gift, your own personal devil notwithstanding.