The sobriquet of Tulkas (S/28), a masculinized form of the adjective astalda “strong” (PE17/115).
Conceptual Development: In the earliest Lost Tales, the sobriquet of Tulkas was ᴱQ. Poldórea (LT1/79), which was the adjective ᴱQ. poldórea “muscular” used as a name (QL/75). The name ᴹQ. Poldórea still appeared in Silmarillion drafts from the 1930s, variously glossed “Strong One” or “Valiant” (SM/79, LR/206). It appeared in The Etymologies as an adjectival form of ᴹQ. poldore “physical strength; might” from the root ᴹ√POL(OD) “physically strong” (Ety/POL).
In later writings, the name Q. Poldórëa appeared in Tolkien’s Words, Phrases and Passages from the Lord of the Rings from the 1950s, along with variants Poldor and Poldorno, where he considered a new meaning for this name as “breaker up of the hard/tough” with its second element coming from the root √DOR “hard” (PE17/181). Ultimately, though the name was changed to Astaldo in Silmarillion revisions from the 1950s-60s (MR/149).
The unglossed root ᴹ√STALAG appeared in The Etymologies of the 1930s with derivatives like N. thala “stalwart, steady, firm” and N. thalion “hero, dauntless man” (Ety/STÁLAG), the latter a sobriquet of Húrin typically translated as “Steadfast” in the narratives themselves (S/199). Similar forms appeared in Early Noldorin Word-lists from the 1920s derived from the primitive form ᴱ✶stalga (PE13/153).
The root √STAL “strong” was mentioned in passing as the basis for the adjective Q. astalda in a rejected page associated with roots having to do with “large & small” words, probably from the late 1960s (PE17/115; VT47/26 note #26). The name Q. Astaldo “Valiant” appeared as a sobriquet of Tulkas in later versions of The Silmarillion (S/28), replacing the earlier name Q. Poldórëa of similar meaning (MR/146, 149; LR/206). For purposes of Neo-Eldarin, I would assume √STAL (and its derivatives) means “✱valiant” rather than “strong”.