istupiti

1. (+ iz + gen.) to leave; to withdraw from
   Istupio je iz stranke posle izbora. — He left the party after the election.
   Zemlja je istupila iz sporazuma. — The country withdrew from the agreement.
2. (in public) to speak out; to come out with a statement
   Ministar je javno istupio i izvinio se. — The minister spoke out publicly and apologised.
3. (from a line, rank) to step forward
   Jedan vojnik je istupio iz stroja. — One soldier stepped out of the line.
4. (a knife, blade) to blunt; to dull
   Istupio je nož sekući kosti. — He blunted the knife cutting bones.
   Sekira se istupila od dugog rada. — The axe got dull from long use.

In the senses 'leave', 'speak out' the imperfective is istupati. In the sense 'blunt' (stressed istúpiti) it is a different verb, derived from tup; its imperfective is tupiti.