The verb for “to eat” derived from the root √MAT of the same meaning (VT39/7).
Conceptual Development: This verb and root are quite well established, dating all the way back to ᴱQ. mat- and ᴱ√MATA of the Qenya Lexicon of the 1910s (QL/59) and appearing as ᴹQ. mat- and ᴹ√MAT in The Etymologies of the 1930s (Ety/MAT), as well as numerous other places albeit with occasional variants like mata- (PE12/26). This verb was one Tolkien often used in examples of verb conjugations and as such its inflected forms changed considerably over time, but that is more a topic of the evolution of the Quenya verb system itself.
mat- (1) vb. "eat" (MAT, VT45:32), also given as mata- (VT39:5), pa.t. mantë "ate" (VT39:7). The form matumnë is said to be future-past: "was going to eat", with the "OQ" (Old Quenya?) future-past element umnë (VT48:32; possibly this could function independently as a form of the verb "to be", hence "was to be"). It is not clear if the form matumnë is itself "Old Quenya" as if this is an archaic future-past formation, or it is just umnë (as an independent word) that is archaic. (Note: Tolkien's translation of matumnë is actually "I was going to eat", but the pronoun "I" does not seem to be expressed in the Quenya form.) Adj. or pseudo-participle #matya "eating" in melumatya "honey-eating" (PE17:68)