"Are you traveling?"
The verb is kusafiri "to travel" and the noun is safari "a trip, journey".
These are loanwords from Arabic and Arabic has a pretty complicated system of deriving words from each other not by just adding prefixes or suffixes but by changing (or adding/subtracting) the vowels between three consonants, as well as using prefixes and suffixes. You'll see a lot of Arabic loanwords in Swahili that have different vowels. This is a good example. Here's another.
ra = ministry
ri = minister
And quite a big family of words comes from the Arabic root k-t-b which has to do with writing. Here are some examples.
bu = book (the Arabic is kitab, with the plural being kutub, but in Swahili, the plural is formed the Bantu way: vitabu)
ba = library
ba = text; constitution; declaration
bu = clerk, secretary
ba = contract
So, basically, sometimes you might notice funny changes in the vowels in related words and if that happens, you're probably dealing with loans from Arabic.
That's a very interesting and helpful comment on an aspect of the relationship between Kiswahili and Arabic. Thanks. I wonder why DL isn't, at the present, accepting "Mnasafiri?" which is an equally legitimate choice as "Unasafiri"? when the singularity or plurality is left to our imagination.