The Slavic languages have almost no native words containing /f/, which did not exist in Proto-Indo-European (PIE). It arose in Greek and Latin from PIE *bʰ (which yielded Slavic /b/). In some instances in Latin, it represented historical th-fronting and derived from Proto-Indo-European *dʰ, and in the Germanic languages from PIE *p, which remained unchanged in Slavic. The letter ф is thus almost exclusively found in words of foreign origin, especially Greek (from ph and sometimes from th), Latin, French, German, English, and Turkic.
The few native Slavic words with this letter (in different languages) are examples of onomatopoeia (like Russian verbs фукать, фыркать etc.) or reflect sporadic pronunciation shifts:
from пв/pv/: Serbian уфати 'to hope' (cf. Church Slavonic уповати 'to hope')
from хв/xv/: Macedonian сфати '(he) understands' (cf. Church Slavonic схватити 'to take, to catch')
from х/x/: Russian toponymФили 'Fili' (from хилый 'sickly')