peniti

1. (peniti se) to foam; to froth; to lather
   Pivo se previše peni. — The beer is foaming too much.
   Ovaj šampon se slabo peni. — This shampoo doesn't lather well.
2. colloq. (peniti se, of a person) to fume; to be seething
   Šef se penio kad je čuo za grešku. — The boss was fuming when he heard about the mistake.
3. (+ acc.) to froth
   Aparat sam peni mleko za kapućino. — The machine froths the milk for cappuccino by itself.

In the senses "to foam" and "to fume" it is usually used with se, but also occurs without it (šampon dobro peni, peniti od besa).