A root variously meaning “blessed, fortunate, prosperous” (PE17/146) and in some circumstances “healthy” and “good (physically)” (PE17/149, 172) or just simply “good” (PE17/146, 150, 153, 158). It was first mentioned in The Etymologies of the 1930s in association with the root ᴹ√ALAM “elm” to which it might be related “since the elm was held blessed and beloved by the Eldar” (Ety/ÁLAM). In Quenya at least its sense was influenced by √GAL “grow, be healthy, flourish” (PE17/146, 153). At one point Tolkien said “this stem was less used in Sindarin, but occurs in a few old forms”, notably S. elia- “to cause to prosper, bless” and S. alw [alu] “wholesome (PE17/146).
The existence of this root is one of reasons that Tolkien decided to abandon la-negation around 1959, saying “AL, LA have too much to do”. Tolkien’s vacillations on the nature of la-negation might therefore have pushed this root in and out of favor, but it is difficult to tell for certain.