alacer.

Somewhat uncertain. The masculine nominative singular form is derived by syncope from alacris which probably developed by dissimilation from pre-Latin *alaklis, with the ending ultimately going back to *-tlis, from *-tl-, a variant of the zero-grade of the agent suffix *-tḗr. Compare volucer and volucris. However, De Vaan regards the origin of the *-kris ending as uncertain. The *ala- portion is seemingly from a root of the form *h₂elH-. De Vaan and Schrijver consider ambulō (“to traverse, walk”) a potential but not certain cognate, and reconstruct the common root (if the relationship is valid) as Proto-Indo-European *h₂elh₂- (“to wander, roam”), cognate with Ancient Greek ἀλάομαι (aláomai, “roam”). The same root may alternatively be reconstructed as Proto-Indo-European *h₂el-. Another possible cognate is Ancient Greek ἐλαύνω (elaúnō, “to drive”), from Proto-Indo-European *h₁elh₂- (“to drive”). Another theory connects the word to Proto-Germanic *aljaną (“vigor, strength”), whence English ellen.