Qenya
at(a)-
prefix. back, again, re-
tanta
adjective. double
atta
cardinal. two
opto
noun. back
waryallume
adverb. *at a different time
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A word (noun?) translated as “back” in notes on The Feanorian Alphabet from the 1940s, given as the name of the tengwa Q (PE22/50).
Conceptual Development: The Qenya Lexicon of the 1910s had a noun ᴱQ. ponte “back, rear” under the early root ᴱ√POT-I (QL/75). Early Noldorin word-lists of the 1920s had ᴱQ. ponte as the equivalent of ᴱN. bón “back” (PE13/139), while Early Qenya word-lists from this same period had ᴱQ. ponte “back” (PE16/136).
Neo-Quenya: In Tolkien’s later writings, the root √OPO was used for “in front” (PE22/168; VT49/32), so I would replace these early “back” words with a neologism ᴺQ. catta “back” based on the later preposition Q. ca(ta) “behind, at back (of place)”.