aslope

From Middle English aslop, aslope (“at an angle or slant; aside”); further etymology uncertain, probably either: * from Old English āslopen (“slipped away”), the past participle of āslūpan (“to slip away”), from ā- (prefix meaning ‘away; from; off; out’) + slūpan (“to glide, slip”) (from Proto-West Germanic *sleupan (“to move stealthily, creep, sneak”), from Proto-Germanic *sleupaną (“to move stealthily, creep, sneak”), probably either from Proto-Indo-European *slewbʰ- (“to glide, slide, slip”) or *slewp- (“to glide, slip”)); or * from Middle English a- + *slope (adjective). However, the Oxford English Dictionary notes that the adjective slope (now obsolete) is attested later than aslope and is probably an aphetic form of the latter. The adverb is more common than the adjective.

来源：wiktionary