Related Clues:

• Lacking a partner
• Unmatched, as a sock
• Like 1, 3 and 5
• Matchless, e.g.
• Unlike 666
• Like all but one prime
• Prompting a 'hmm,' perhaps
• Like all prime numbers except 2
• Like 1, 3, 5, 7 ...
• Really strange
• Like 9 or 5
• Like 1, 11, 111, and 1,111
• Like an unpaired sock
• One-sided
• Abnormal