early 14c., ajournen, "assign a day" (for convening or reconvening), from Old French ajourner (12c.) "meet" (at an appointed time), from the phrase à jorn "to a stated day" (à "to" + journ "day," from Latin diurnus "daily;" see diurnal).
The sense is to set a date for a re-meeting. Meaning "to close a meeting" (with or without intention to reconvene) is from early 15c. Meaning "to go in a body to another place" (1640s) is colloquial. The unhistorical -d- was added 16c. Related: Adjourned; adjourning.