Related Clues:

• 'That's ___ ...'
• Freakish
• Quite strange
• Unpaired
• Missing a few marbles
• Kind of duck
• Like all primes but one
• Lacking a match
• Atypical
• Quite unusual
• Unmatched, as socks
• Out of the ordinary
• Like Oscar and Felix's pairing
• Not quite right
• Unsteady