From the scholia to Aristophanes, Birds 662, where the headword, a noun which can be either masculine or neuter, occurs (web address 1). Identified in LSJ s.v. as Carex riparia, sedge, a reed-like grass which grows on riverbanks and in marshy areas; but Dunbar 420, without mention of this, equates it with Butomus umbellatus, the sharp-edged leaves of which can indeed "cut oxen" (the literal meaning of boutomos) on the mouth or tongue.