someone or something that could change a situation in a way that you do not expect.

The independent candidate is the joker in the pack in this election.

• a body blow
• a bolt from the blue
• a bone of contention
• a bone to pick with
• a bone to pick with someone
• a bon mot
• a bon viveur
• a bossy boots
• a bottomless pit
• about ears
• about person
• about someone's ears
• about time
• About time too!
• about to
• about your person
• above all
• above all else
• above and beyond
• above and beyond something
• above and beyond the call of duty
• above suspicion
• above the fray
• above the law
• a brain box