The verb G. masta- “feed, graze” appeared in the Gnomish Lexicon of the 1910s (GL/56). Its etymology is unclear; it is tempting to assume it is derived from the early root ᴱ√MATA “eat” (QL/31), but is not clear how the st would arise in the phonology of Gnomish. Another possible root is ᴱ√M(B)ASA, but in the Qenya Lexicon this root was glossed “cook, bake” (QL/59, 63).

Neo-Sindarin: This word can easily be adapted as Neo-Sindarin ᴺS. masta- based on an ancient causative verb = √MAT “eat” + ✶-tā, since t + t from suffixion resulted in st in the Elvish phonology of the 1930s-60s. I would limit this verb to the sense “[put to] feed, graze” for animals, while directly feeding another would be ᴺS. media-.