A rejected root in The Etymologies of the 1930s glossed “dig” (Ety/DAG), it may have been a transient replacement for √SAP.
Middle Primitive Elvish
tug
root. *muscle
dag
root. dig
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
An unglossed root in The Etymologies of the 1930s with derivatives like ᴹQ. tuo/N. tû “muscle, sinew; vigour, physical strength” and ᴹQ. tunga/N. tong “taut, tight (of strings, resonant)” (Ety/TUG). It was also the basis for the name N. Tuor “strength-vigour”, the only place where this name was given a clear etymology, though later Tolkien decided this name was actually from the language of the Edain (PM/348, 364 note #49).