This word appeared in notes from the late 1960s, where Tolkien said that it was “translated ‘smith’, but meaning a craftsman in wood, stone, or metal: carpenter (carver), mason (sculptor), or [metal] smith (PE17/107)”. Tolkien further gave sintamo specifically for “[metal] smith”, but said “tamo unspecified, especially among the Noldor, was usually employed = sintamo, our ‘smith’ (PE17/108)”. In this 1960s note the word tamo was derived from the root √TAM “construct” (PE17/107).
Based on its use as an element in other words, tamo could be translated as “smith, builder, wright, artificer” = “✱one who crafts things from some material”. As a reduced element in compounds it becomes -tan, since final m became n in Quenya (PE19/104). This can be seen in words like calmatan “lampwright” (PE17/96) and ciryatan “shipbuilder” (S/265). The element tamo can also be seen in one of the names for Aulë: Martamo “World-artificer” appearing a very late note according to Christopher Tolkien (LT1A/Talka Marda).
Conceptual Development: Early Qenya Word-lists of the 1920s had ᴱQ. talka “smith” (PE16/138) as in ᴱQ. Talka Marda “Smith of the World” from the early Lost Tales (LT1/180). This 1920s word was probably a cognate of ᴱN. taglon “smith” (PE13/153) and was likely based on the early root ᴱ√TAKA “fix, fasten”. A remnant of talka “smith” can be seen in primitive ᴹ✶talkō “craftsmen, wright”, which appeared in the first version of Tengwesta Qenderinwa (TQ1) from the 1930s as a derivative of ᴹ√TAK (PE18/54).
The Etymologies of the 1930s had ᴹQ. tano “craftsman, smith” derived from primitive ᴹ✶tanō under the root ᴹ√TAN “make, fashion” (Ety/TAN). In this document Aulë’s name was ᴹQ. Martan(o) “Earthbuilder, Earth-smith” (Ety/MBAR, TAN). Remnants of this √TAN derivation can be seen a few later names like Q. Artano “High-smith” (UT/254), but it seems Tolkien decided sometime in the 1960s to make the root √TAM instead. Some of the 1950s and 60s compounds with -tan were probably originally from √TAN, but these remain fine after the change in root since final m became n as noted above.
-tar or tar-, element meaning "king" or "queen" in compounds and names (TĀ/TA3), e.g. Valatar; compare the independent nouns tár, tári. Prefix Tar- especially in the names of the Kings and Queens of Númenor (e.g. Tar-Amandil); see their individual names (like Amandil in this case), cf. also Tar-Mairon "King Excellent", title used by Sauron (PE17:183). Also in Tareldar "High-elves"; see also Tarmenel.