tupiti

1. (of a blade) to blunt; to make dull
   Ne seci konzervu nožem, tupiš ga. — Don't cut the can with the knife, you're blunting it.
2. (of senses, mind) to dull; to weaken
   Stalni stres tupi pažnju. — Constant stress dulls your attention.
3. colloq. (of talking) to go on and on; to drone on
   Ne tupi više o tome, shvatio sam. — Stop going on about it, I get it.

In colloquial speech "ne tupi!" means "stop droning on!".