WordWeb Online

1. Short underpants for women or children (usually used in the plural)
   - pantie, panty, scanty

1. Get involved, so as to alter or hinder an action, or through force or threat of force
   "Why did the U.S. not step in earlier in WW II?";
   - intervene, interfere, interpose
    
2. Act or speak as a replacement (for someone or something)
   - substitute, deputize, deputise [Brit], sub

Derived forms: stepped in, steps in, step-ins, stepping in

Type of: interact, pants [Brit], replace, supercede [non-standard], supersede, supervene upon, supplant, underdacks [Austral], underdaks [Austral], underpants, undershorts [N. Amer]