This document is intended to remedy this situation, by showing some nasty words and expression that's been used or suggested in the past, together with some of my own coinings.
Disclaimer: the author is not a fluent Lojban speaker (as yet, there are none). He hasn't even been with the Lojban movement for more than one year. Thus, there may be semantic or syntactic inaccuracies in some of the examples.
Expletives if this kind don't easily fit into Lojban, because of the aforementioned cultural neutrality. kalci merely means "excrement from living beings", and not "shit!". kalci is something you would say if somebody pointed at a pile of dung and asked what it was. "It's shit."
Luckily, Lojban has a set of words that are called "attitudinals", or "pure emotion words" that can do the job of expressing emotions such as anger, joy, love, fear and expectation. One of these words, oi, expresses anger, but it has nothing to do with the ordinary statements (bridi) of Lojban (they can, however, be used to convey the feelings one has about the meaning of whole or parts of the sentence; see the grammar for details about this). It is not translated "I'm angry", but something like the English "arrgh", "grr", or speaking with an angry voice. So, is this the word we're looking for? Not quite. One thing is that it's not vulgar, it's never bad etiquette to use it. (When did you last hear anyone saying "Fuck this!" in a business meeting?) Another thing is that when used in conjunction with a sentence, it translates as saying the sentence (or certain words of it) with an angry voice. Using words instead of prosodic differences was deliberately chosen, because:
Some Lojbanists have called each other names using doi, the vocative particle. So they say "doi bakni", which means "Cow!" (as in "Oh, cow! I'm talking to you!"). The intended meaning is, of course, "You cow!", but it is not known for sure whether doi can be used that way. A theory is, that if there are five people and a cow in the room, and one of them says "doi bakni", the rest of the four people would be bound to believe she had gone crazy and started to talk to the cow. If the only living things in that room were those five people, they couldn't be sure who she were talking to (unless some of them, previously, had been nick-named "Cow").
Let's stick with this example for a little bit more. There still is this guy we sincerely hate, and we want to call him a cow. We could of course say "do bakni" ("You are a cow."), but Lojban being a logical language, with no room for metaphors, the sentence would be a lie (unless the insultee is actually, in a strict, biological sense, a bovine).
The approach I recommend (for some, but not all cases), is using the words simsa, which means "similar" or "like". We would say "do simsa lo'e bakni", which means "You are like a typical cow" (we could have chosen other articles here, but for obscure, technical reasons, these are not so useful in these case). The property the insultee and the typical cow has in common, is left unspecified. Perhaps it is the intelligence (very insulting), or the capability of producing milk (not so insulting) or the size of the tongue (ridiculous).
You could also use simlu, which means "appear" or "seem". You could say "do simlu le ka bakni". It means "You look like a cow", "You behave like a cow", or "You seem to be a cow".
Substitute any brivla for the word "bakni" in these examples. (If you don't know what a brivla is, check with the grammar).