The editor suggests the word is probably an error, but offers no alternative. The same text is published in Five Med MSS 47.C205, where the word is interpreted as a deviant spelling of pume.
s.
1(?):
(
MS: c.1325
)
E checun jour pernez celez erbes en servoyse auxi gros cum un pombe petit dedeins cele servoyse e bevés checun jour un tret a vespre
138.561
pombe (c.1325)