This was the word for “bread” in Sindarin and its conceptual precursors for much of Tolkien’s life, derived from the equally long-lived root √MBAS “bake”. The word dates all the way back to the Gnomish Lexicon of the 1910s where G. bast “bread” was derived from the early root ᴱ√M(B)ASA “cook, bake” (GL/22). ᴱN. bast “bread” reappeared in Early Noldorin Word-lists of the 1920s (PE13/138) and appeared again as N. bast “bread” in The Etymologies of the 1930s under the root ᴹ√MBAS “knead” (Ety/MBAS).
It appeared without a final t in the Sindarin version of the Lord’s Prayer from the 1950s: anno ammen sír i mbas ilaurui vín “give us this day our daily bread” (VT44/21). However the t was restored in the phrase penim vast “we have no bread” from around 1959 (PE17/144). The late vacillations on the presence and absence of t are likely connected to Tolkien challenges with the derivation of lembas; see that entry for discussion. Likewise, the mutated forms mbas vs. vast indicate some late uncertainty on whether the primitive form began with mb- or b-.
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume the normal form was bast “bread” from ancient mbasta, so that lenited forms show mb-.
soft mut. of #bast.