1. Without warning
   - all at once 
2. Happening unexpectedly
   - suddenly, of a sudden

Encyclopedia: All of a sudden

• implosive
• impluvium
• imply
• impolite
• impolitely
• impoliteness
• impolitic
• imponderable
• import
• import barrier
• import credit
• import duty
• importable
• importance
• important
• important person
• important-looking
• importantly
• importation
• imported
• importee
• importer
• importing
• importunate
• importunately