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"Frack" is a Colonial expletive, roughly analagous to "shit" or even the milder "rats" or "darn" of the Original Series. Its subversive value, exploited far more extensively in the Re-imagined Series, is that it sounds like a variant of "fuck", and in the latter series it actually conveys that meaning.
There are two main ways to spell the term, but "frack" is how it appears in the Writer's Guide, dated October, 2, 1978.
With the exception of the Galactica 1980 episode "The Return of Starbuck", it is used solely in the Original Series episodes and publications relating to that series. For the first nine episodes of Galactica 1980, the term "felgercarb" is used in lieu of "frack", likely because of the "Kiddie Hour" timeslot 1980 held at the time.
Usage by episode