I don’t want to drip feed so this might be a bit long sorry!
A little background for starters. I work for a large Italian company in the U.K. in a male dominated industry. The company has a number of ‘sub-companies’ which operate separately on different activities but which are often closely linked.
The company I work for owns our office building and one other of the Group companies ‘rents’ some floors in the building.
On Tuesday I was carrying out a task which involved checking the fire extinguisher types and locations throughout the whole building. All areas of all floors are accessed using key-cards and only people from the relevant company/project have access to each area.
One of our security guards, Bob, came with me to walk through the whole building top to bottom te ensure I could access all areas.
As I entered one of the areas used by the other company I paused to check my plans just outside the office of Mario whom I know, Mario gives me a cheery hello and I then continued to look at the plans to work out where to check.
Another employee let’s call him Luigi, walked past, and, nodding his head at me said to Mario “bordello”. Mario looks up from his work and says “eh?”. Luigi again nods at me and says again “bordello” before walking off.
I don’t speak much Italian, and in the moment wasn’t sure I had understood so quickly checked what I needed to and left the room. At which point Bob says to me with an eyebrows raised face “even I know what that word means”.
I felt a bit uncomfortable but still wasn’t sure what it meant so sort of brushed it off.
Yesterday I found it was still bothering me, so I looked it up, and bordello means brothel or whorehouse. Obviously this is very insulting/offensive.
In the male dominated environment I work in sometimes the ‘locker room” environment means people overstep lines, plus there is sometimes a linguistic/cultural difference and I have never before made a complaint. This is because A) all things a bit 
that have been said before I know were intended complimentarily/playfully and were just one party going a bit far or wording things in a way that comes across wrongly.
And B) anything which may have previously happened has happened with someone with whom I have a good working relationship and with whom I have no problem simply saying “no, that’s a bit far” or “you can’t say that” etc. No-one has ever ‘repeat offended’.
However I have lodged a formal complaint about this because I have never even spoken to Luigi AND there is no way at all that what he said could have been anything other than an offensive insult.
I mentioned this to a colleague who then asked what happened and their response was “it’s just a word”.
So, was I being unreasonable to have made a complaint?
*names have been changed throughout 