Hi Gav,
Following David's and Bill's introductions, i would add that it appears to be like that shampoo ... "two in one"
The word has a number of variations and is a corruption of Frank (certain European peoples ) and basically means foreigner.
It was used in the derrogatory sense, with a racist connotation.
On the other hand, as Portuguese were the first foreigners in most of those places, Firangi would also or implicitly mean Portuguese.
In colonial Sry Lanka, "Parangi" was associated to both Portuguese people and a form of syphilis introduced by them
.
A tourist spot in Malaysian Penang is called "Batu Feringgi" as once visited by the Portuguese fleet ... no syphilis version this time
Fernando