1. In a way that was not expected
   - out of the blue 
2. Without advance planning
   - by chance, accidentally, circumstantially

See also: unexpected

• background processing
• background radiation
• background signal
• backgrounder
• backgrounding
• backhand
• backhand drive
• backhand shot
• broken-backed
• broken-down
• broken-field
• brokenhearted
• brokenheartedly
• brokenheartedness
• brokenly
• brokenness
• broker
• broker-dealer
• brokerage
• brokerage account
• brokerage firm
• brokerage house
• broking
• brolga
• brolly