late 15c., "about to happen or appear," present-participle adjective from Middle English forthcomen, from Old English forðcuman "to come forth, come to pass;" see forth + come (v.). Meaning "informative, responsive" is from 1835, via the notion of "in such a position or condition, as a person or a thing, that his or its presence when needed can be counted on." A once-common verb formation; English also had forthbring, forthcall, forthdo, forthgo, forthpass, forthset, all now obsolete.