Robinhj Sapper is not a made up word, you should find it in the dictionary; my grandmother even had a dog named 'Sapper'. Originally in medieval times they were the people that dug tunnels and planted explosives to collapse castle walls etc. The verb was 'To sap'. In later wars they were the branch of the army that laid or cleared minefields wholesale (as opposed to Bomb Disposal), built or laid temporary bridges and other 'Engineering' duties. In the British Army A 'Sapper' would be in the Royal Engineers but I believe the US Army has them as individual members of Light Combined Arms teams eg alongside Rangers. British Commandos would also have combat engineering specialists as part of a team but they would not call them 'Sappers' as far as I know.
TC I agree with Robin. It's a disused noun of which today's equivalent is "combat engineer." Their job is munitions/demolitions/sabotage, among other things. The Sappers in Erikson's books deal mainly in mayhem with moranth munitions and are generally regarded as being a little (Fiddler) or a lot (Crumb) crazy.
Snaz Hound I am an engineer as well as an retired infantry officer, in the military "sapper" is the rank and thus position of a trained combat engineer ordinary rank and file (OR) member. Sappers are known presently and throughout history as the soldiers whose job is to build, support and maintain combat engineering systems, which include everything from bridges, fortifications, trenches, munitions et al. My preference was to be a combat engineering or communications officer, but I was referred to the infantry. It would have been impossible to do my role as an infantry officer without the incredible support that combat engineers gave. I still have a warm fuzzy feeling for all combat engineers be they officer or ordinary rank and file. It is obvious that Steven Erikson respects the role that combat engineers (thus sappers) perform. Personally I love the role that sappers perform in the Malazan army.