zaklinjati

1. (zaklinjati se) to swear; to vow
   Stalno se zaklinje da više neće pušiti. — He keeps swearing he won't smoke anymore.
   Zaklinjem se da nisam ništa znao. — I swear I didn't know anything.
2. formal (+ acc.) to implore; to beg
   Zaklinjem te, reci mi istinu. — I beg you, tell me the truth.

In the sense 'to swear' it is used with se; the perfective is zakleti se.