Frak
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Definition
Frak is an expletive analogous to the English word fuck.
Usage
It is used much like its English counterpart, i.e. "Frakkin' rats."/"Fuckin' rats."
It is a multipurpose and adaptable term, for it is capable of being used as a term describing the act of sex (ex: Tyrol just frakked Sharon!) or can be used to denote one's hatred of something (ex: Frakkin' Cylons).
In the original series, it was spelled "frack".
The meaning and usage is similar to the military slang term "frag" (also "fragged" etc., originally short for "fragmentation grenade") - supposedly originating in the Vietnam War Era. The episode title Fragged (2.03) suggests that maybe "frak" is just a misspelled version of "frag".