Losing a limb in a dream often symbolizes a loss of capability -- a skill, role, or sense of independence you're afraid is being taken from you. It can surface during a major life change, when you feel less equipped to handle what's ahead than you did before that change began.
Mercury being strong and in proper places of the Heavens, well configurated of other stars or planets in nativities, denotes convenient dignity to the native thereby; but Mercury of himself is a planet weak. - William Lilly (1602.-1681.)