It’s about time. We are now on Instagram!
1650s, "learned preamble to a book," from Greek prolegomenon, noun use of neuter passive present participle of prolegein "to say beforehand," from pro- "before" (see pro-) + legein "to speak" (see lecture (n.)) + suffix -menos (as in alumnus). The same sense is in preface (n.). Related: Prolegomenary; prolegomenous.