1650s, originally theological, "matters of practice," as opposed to credenda "things to be believed, matters of faith," from Latin agenda, literally "things to be done," neuter plural of agendus, gerundive of agere "to do" (from PIE root *ag- "to drive, draw out or forth, move").
The modern sense of "items of business to be done at a meeting" is attested by 1882. "If a singular is required (=one item of the agenda) it is now agendum, the former singular agend being obsolete" [Fowler].