1. In an exhaustive manner
   "we searched the files thoroughly"
   - exhaustively 
2. Completely and absolutely ('good' is sometimes used informally for 'thoroughly')
   - soundly, good

See also: thorough

• instable
• instal
• install
• installation
• installation charge
• installer
• installing
• installment
• installment buying
• installment credit
• installment debt
• installment loan
• installment plan
• installment rate
• instalment
• instance
• instancy
• instant
• instant coffee
• instant replay
• instantaneity
• instantaneous
• instantaneous sound pressure
• instantaneously
• instantaneousness