Green’s Dictionary of Slang

ally slope n.

In phrases

do an ally slope (v.) (also do an alley) [the comic character Ally Sloper, ‘a seedy proletarian loafer’, featured in Alley Sloper’s Half-Holiday, publ. by Dalziel Bros., 1884–1923 and illus. (at first) by W.G. Baxter, an ex-patriate American. Note WWI milit. use Alley Sloper’s cavalry, Army Service Corps; ? underpinned by SE slope off + Fr. aller, to go]

to escape, to make off.

1925		E. Jervis 25 Years in Six Prisons 59: Meanwhile the ‘con’ man and his son would ‘do an Ally Slope’ through another door.
1937–84		Partridge DSUE (1984) 317/2: do an alley. To depart; to hurry away.