Since segment \( AD \) bisects angle \( CAB \), we know that:
\[ m\angle CAB = m\angle BAD + m\angle DAC \]
Because \( AD \) is the angle bisector, \( m\angle BAD = m\angle DAC \). Let’s denote \( m\angle BAD \) as \( x \). Therefore,
\[ m\angle CAB = x + x = 2x \]
Given that \( m\angle BAD = 21^\circ \), we have:
\[ x = 21^\circ \]
Thus:
\[ m\angle CAB = 2x = 2 \times 21^\circ = 42^\circ \]
Therefore, the measure of \( \angle CAB \) is \( \mathbf{42} \) degrees.