1. To act below one's abilities; to do less than is requisite.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• underdow,
• underdoo,
• ynderdo,
• hnderdo,
• jnderdo,
• inderdo,
• 8nderdo,
• 7nderdo,
• ubderdo,
• umderdo,
• ujderdo,
• uhderdo,
• unserdo,
• unxerdo,
• uncerdo,
• unferdo,
• unrerdo,
• uneerdo,
• Undgrdo,
• underdos

The adverb form of "underdo" is "underdone".