1. Impossible to achieve
   - unattainable, undoable, unrealizable, unrealisable [Brit]

See also: impossible, unachievably

• unfrequented
• unfretted
• unfriended
• unfriendliness
• unfriendly
• unfrightened
• unfrock
• unfrosted
• unfrozen
• unfruitful
• unfueled
• unfuelled
• unfulfillable
• unfulfilled
• unfunctional
• unfunded
• unfunny
• unfurl
• unfurnished
• unfurrowed
• ungainliness
• ungainly
• ungallant
• ungarbed
• ungarmented