There's a trouble with language.  Especially with historical crime fiction.

When a story is set in a historical period, there are two things that authors have been given the "warning" about. 
Firstly it's anachronisms.  These are easy to make and the first thing that critics select to criticise.  You don't want the errors - obvious when pointed out - that come from bad research.  To put it bluntly, you don't want a Roman to receive a telegram!  Anachronisms like this are a bummer and, when you spot 'em, they're difficult to get past.  Then again, a good author - who's done their research - shouldn't fall at this hurdle.  Let's face it, if you want to write a novel set in a historical period then you should know, at least in part, the setting in which you write.

The second "warning" given to authors is about "voice" and this is where I must nail my colours to the mast.

We look at history.  We - hopefully - know (roughly) what went on.  We are - I assume - intelligent folk.  Thus do we honestly expect an Elisabethan character NOT to swear, using a ... ahem ... base phrase such as "He couldn't give a fuck!"?  Now, I don't want to get into the derivatives of swearing but the use of such in writing a believable work of fiction is important.
We, as readers, are made to enter a past time - a past life, if you prefer.  We swear (as much as we'd like to think we don't) and so we must assume that our ancestors did too.

People are people.  Around the world and, I think, obviously throughout the years.  We use phrases, expressions, idioms that (from the outside) sound nonsensical.  But, in the context of the world they live in, some phrases are perfectly understandable.

LUPUS IN FABULA

Literally, in Latin, this means "The wolf of the story".  The idea is that two people are gossiping and the subject of their gossip walks in, restricting further tales.  One says "Ah, here is The Wolf of The Story!"  The language and setting is "foreign" - Latin - but the phrase and the concept is easy.
So why is it so hard for "purists" to not baulk against slang or "less than classical"?  Do they want writing to be set in Roman times and the characters sound like Shakespeare on his most priggish?  Our playwrights - while clever - were writing plays.  They were not writing verbatim what was said.  If, say, the hero Leander turned around and said "I've got to go to the privvy!" it's not what would've been recorded.  This was not real - this was manufactured.
So where does this leave us with our modern interpretation?  Should authors only use phrases or idiom only "found" for that period or can it be assumed - in my own interpretation - that "native" phrases can replace that which is read?

   Where an author writes (a quote):  "How  else would  you describe something that waits for the perfect psychological moment and then pisses all over your living-room floor?  Jupiter, even a Gallic wolfhound wouldn't do this much damage.  For a parallel you'd need to go the length of a fucking elephant."

Firstly - just because we're not with the idea that a Roman ( of Tiberius' reign) would use the phrase "psychological", doesn't mean they couldn't have the concept.
Secondly - Using phrases such as "pissing" and "fucking" were perhaps not polite but used.
Thirdly - There is a lot of evidence to show that the Romans used Gallic Wolfhounds, now known as Irish Wolfhounds.  As an owner of one, I can attest that they can - in play - cause a lot of damage to furniture and so on.

Thus, the above quote - while sounding too modern - is, actually, perfectly acceptable.  Apart from those who  think all Romans should talk like great orators.  Who need to get out more.