Thumb
The opposable digit of the hand — the feature that makes grip and tool use possible.
A thumb in a dream is the dream's image of the feature that makes the human hand into a tool-using hand. The thumb is what opposes the other fingers; without it the hand can wave, not grip. The dream brings the thumb when something in your life — a small capacity, a key skill, a particular person on a team — is the specific element that turns a set of capable parts into a working unit. A thumb in good use is the dream affirming the essential piece. A thumb injured, the rest of the hand suddenly clumsy at everything, is the dream noting that the small specific feature was doing more work than its size suggested, and its absence is going to be felt in every task.