A common phrase within investment banking circles for a software developer with limited or zero true commercial capabilities. Mounties are often also considered to have poor communication skills and questionable hygiene practices.
On rare occasions, a Mountie can be a British national. This is rare (see Urban Dictionary definition Rare as Rocking Horse Shit). More commonly, a Mountie comes from developing countries and receives sub-standard training in the UK.
"Can you write me a 'hello world' example in C# please?"
"I'm afraid that's beyond my abilities, I'm a Mountie"