slope: [15] The noun slope did not emerge until the 17th century. Originally it was an adverb, short for the now defunct aslope. This is generally supposed to go back to an unrecorded Old English *āslopean, an adverbial use of the past participle of āslūpan ‘slip away’. Such a scenario would appear to fit in well with the colloquial slope off ‘leave’, but in fact this usage did not emerge until the early 19th century, in America.
