There's something sly about sloping away. Something slippery and slouching and sliding and sleuth-like. Something almost slithery; and definitely something slinking.
Why do so many of those rather similar words begin sl? There must surely be a reason. I've no idea what it is, myself, but they're lovely words to say, anyway.
To slope away (or off). This word is a shortened form of the old English word aslope, which is related to the word āslūpen, from slūpen, to slip.