Like many sci-fi/fantasy universes, the Star Wars movies and Expanded Universe feature many invented swear words and insults, which allow writers to make their intended meaning clear while keeping the works PG.
One of the more common sources for expletives in the Star Wars universe is Huttese, the language of the Hutts and -- due to the spread of the Hutts' criminal empire -- one of the most common languages throughout the galaxy. However, many other cultures and alien races have contributed to the wide variety of bad language in the Star Wars universe. Here are just a few examples.
In general, words that start with a "K" or hard "C" sound take the place of the F-word and similar sounding euphemisms in English.
Crink/crinking was a common swear for pirates in the Outer Rim. It first appears in Allegiance by Timothy Zahn.
Farkled is an adjectival F-word euphemism, used in such contexts as "the engine's farkled" or "we got farkled in that deal."
Kark/karking, a Huttese expletive, appears frequently in Star Wars: Legacy.
Kriff/kriffing has a similar meaning, although from context it does not appear to be quite as strong a curse. It first appears in Timothy Zahn's Hand of Thrawn duology, and probably comes from a simple rearrangement of the letters in "fricking," a euphemism for the F-word.
Krong is another swear from the Outer Rim pirates. It doesn't appear to have an adjectival form, but the verb is used in such sentences as "don't krong things up."
Skrog/skrogging appears in Star Wars: Legacy and appears to be of human origin.
Snark/snarking was used by bounty hunters in the Legacy era. It has no relation to the English word "snarky," meaning "snide" or "sarcastic."
Druk is an alien word for excrement and seems roughly equivalent to the S-word. It appears in phrases such as "a drukload of problems."
Holy Sith! is used humorously as a euphemism for its English anagram. (It also appears in a recent Futurama episode.)
Shab is a Mandalorian word for excrement which appears in Republic Commando: Order 66 by Karen Traviss. Shabuir is an insult derived from the word "shab."
Shavit originates on the planet Pakrik Minor and first appears in Vision of the Future by Timothy Zahn. While its exact definition is not stated, we can infer from the similarity to the S-word that it has a similar meaning.
Bantha poodoo, a Huttese phrase meaning "Bantha fodder," first appears in Episode VI: Return of the Jedi as an insult spoken by Jabba the Hutt. While the word "poodoo" sounds like it should translate to a similar-sounding word in English, the EU clarifies the phrase's meaning: Bantha fodder (i.e., food for Banthas) looks and smells disgusting.
E chu ta is another Huttese expletive. It first appears in Episode V: The Empire Strikes Back; although its meaning is not defined, C-3PO exclaims, "How rude!" upon hearing it. It appears to be a particularly insulting and vulgar expression, and is commonly used in Star Wars: Legacy.
Hutt-spawn is an insulting phrase for everyone except Hutts, of course. It appears in Knights of the Old Republic.
Laserbrain suggests that someone is stupid, crazy, or delusional, as in "I don't know where you get your delusions, laserbrain" (Princess Leia to Han Solo in The Empire Strikes Back). Blaster-brained has a similar meaning.
Lurdo is a childish Ewokese insult, roughly equivalent to "dummy." It appears in the Ewoks animated series.
Nerf herder is an insult because shepherds who raised nerfs -- buffalo-like animals native to Alderaan and raised for their meat -- were generally scruffy-looking and smelled bad. Leia throws this insult at Han in The Empire Strikes Back.
Schutta is a Twi'lek insult reffering to women, roughly equivalent to "slut." Its name derives from a weasel-like creature native to Ryloth, the Twi'lek home planet.
Sculag is a Chiss term referring to someone who is weak-minded. It appears in Legacy of the Force: Inferno by Troy Denning.
Sleemo is a Huttese insult which sounds like its English equivalent, "slimeball." It appears several times in Episode I: The Phantom Menace.
Son of a blaster is a rather silly-sounding Star Wars substitute for "son of a gun," which it itself a euphemism for a more explicit insult.
Stoopa (sometimes spelled stupa) is a Huttese noun for a foolish or stupid person.
Vong is a grave insult among the Yuuzhan Vong, implying that someone has lost the favor of Yun-Yuuzhan, the creator god.
Chuba (sometimes spelled chubba) is the Huttese word for "you" or "your." When used as an expletive, however, it can express surprise ("What the chubba is that?") or imply that someone is untrustworthy.
Frizz/frizzled was a common swear among smugglers in the Old Republic. It first appears in Dark Lord: The Rise of Darth Vader by James Luceno.
Shebs is a Mandalorian swear meaning "buttocks." It appears in Republic Commando: Hard Contact by Karen Traviss.
Sithspit is a common curse in all eras of the Star Wars universe, used to express surprise, anger, or other strong emotions. A related curse, sithspawn, refers to creatures genetically engineered by the Sith. Both swears originated on Corellia.
Vape/vaping is a mild expletive originating from Alderaan, roughly equivalent to "damn." Varp is probably a related expletive, used in phrases such as "What the varp!"