Same entry in other lexica, and see also the scholia to Homer, Iliad 2.301 (where i)/dmen [sic] is glossed by the present headword plus gloss).
The older forms of oi)=da have the zero-grade of the root in the plural. In the first person plural, i)/dmen (iota 123) was replaced with i)/smen by analogy with i)/ste, and by oi)/damen (the word used to define the term here) by analogy with oi)=da in Ionic and later koine. Smyth, Sections 794-799 (web address 1).
See also iota 611, iota 612, iota 615, iota 637, iota 680, iota 681, iota 702.