The all- forms derive from Middle French aller, from Old French aler, alier (with subjunctive aill- and other forms with all-), from Vulgar Latin *alō (attested in the 7th century Reichenau Glosses). This has traditionally been explained as deriving from Latin ambulāre via or together with ambler (compare Old Occitan amblar, Italian ambiare, Romanian umbla), but this explanation is phonologically problematic; ambler is additionally more likely a borrowing from Old Occitan. Several theories have been put forth since the 17th century to explain how ambulare could have become aller. Since at least the 18th century, some have suggested that aller derives not from Latin but from Celtic, Gaulish *aliu, from Proto-Celtic zero grade *ɸal-. Compare Welsh elwyf (“I may go”), Cornish ellev (“I may go”), from full grade *ɸel- (see mynd for more). It is also possible to combine the two theories by proposing that the contraction of ambulāre was reinforced by similar sounding forms in Celtic. Franco-Provençal alar and Friulan lâ (“to go”) (compare lin (“we go”), lât (“gone”)) are from the same source, whatever it may be.
From Old English alor, from Proto-Germanic *aluz, *alusō (compare Swedish al, Saterland Frisian ällerboom), variant of *alizō, *alisō (compare Dutch els, German Erle), from Proto-Indo-European *h₂élisos