Why do they say frak in battlestar galactica?

We can assume that, as it follows citizens from the twelve colonies of humanity who haven't lived on Earth, (despite bearing striking similarities to us in other ways) the Word "frak" is their own version of our similar profanity, "f**k".

It is also derived from the original Battlestar Galactica, where the Word "frack" was used as a substitute for "darn" or "s**t, mostly to comply with broadcast decency standards at the time.

Hope that answered your question :)