Related Clues:

• Like 7 or 11
• Offbeat
• Unmatched, like socks
• Lacking a partner
• Unmatched, as a sock
• Matchless, e.g.
• Unlike 666
• Alien
• 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
• Very odd