bidens.

From bi- (“two-”) + dēns (“tooth”).

Ellipsis of bidēns ovis f (“two-toothed sheep”), referring to a sheep of an age (typically between one and two years old) when only the two front incisors in its lower jaw have been replaced by permanent teeth, which are larger, thus standing out in contrast with the six remaining milk teeth (see this Wikipedia article). Compare also victima f and hostia f. Some ancient authors instead believed it was derived from biennis (“two-year-old”), with -d- inserted between the prefix and base and a shift of -nn- to -nt-, neither of which would be regular changes. (Some Latin prefixes, such as re- and prō-, do have variant forms ending in -d- that can appear before a vowel, as in redimō and prōdigō from emō and agō; however, this would be the only example of bid- being used for bi-.) See the quote from Aulus Gellius below, himself referencing already conflicting opinions from earlier authors; and the entry for biennis.

Nominalization of the adjective, perhaps by ellipsis of a phrase headed by a masculine noun such as ligō m (“mattock, hoe”). Compare tridēns m.