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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 [informal]

1. Short underpants for women or children
   - pantie, panty, scanty

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

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