'SLR' was also the British Army's pet name for their version of the
FN FAL. Officially designated the L1A1, it was universally known by soldiers as the 'SLR', which stood for 'Self Loading Rifle'. Until its introduction, the British Army had been armed with the Lee-Enfield
SMLE, a bolt-action rifle (which meant that the soldier had to pull back a bolt in order to load the rifle after each shot). The SLR, on the other hand, 'loaded itself', hence 'self loading rifle'.
In camera terms, a 'reflex' camera is one in which the user focusses whilst looking through the lens - reflexively, as it were, without having to interpret a distance scale. The earliest reflex cameras were TLRs, 'Twin Lens Reflex' cameras, which had two lenses coupled with a gear; one for taking the picture, and one for focussing (as such, a TLR is essentially a clever rangefinder camera).
SLRs date back to the turn of the century and it would appear that there is no 'first ever' SLR; the first modern pentaprism-based 35mm SLR was the 1949 Contax S, examples of which fetch $300 or so on eBay.