"existing, occurring, or appearing at the same time," 1650s, from Medieval Latin simultaneus, ultimately from Latin simul "at the same time," which is related to similis "like, resembling, of the same kind" (see similar).
The Medieval Latin word is perhaps via the extended form simultim "at the same time," or from simul with ending abstracted from Late Latin spontaneus, where the -t- is organic. Related: Simultaneously. "Mark Twain" back-formed from it a verb simultane "do (something) at the same time as (something else)," 1880.
updated on November 05, 2022