1. In an insidious manner.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• insiduously,
• insidiouse,
• unsidiously,
• jnsidiously,
• knsidiously,
• onsidiously,
• 9nsidiously,
• 8nsidiously,
• insidiosly,
• insidiousy,
• insidiosuly,
• insidiously,
• insadiously,
• Incindiously,
• Incedously,
• insideously,
• insodeiously,
• insidioulsly,
• insidiousally,
• insidiousley

• iniquitously,
• iniquitous,
• injudicious,
• incautious,
• in stride,
• injudiciously,
• insidious,
• incautiously,
• insidiousness.

The adjective form of the word "insidiously" is "insidious".

The plural form of "insidiously" is "insidiously". The word remains the same in both singular and plural forms.

The singular form of the word "insidiously" is "insidious"