"to go before," Old English foregan "to go before," from fore- + go. The similarly constructed foredone "killed, destroyed," now is archaic, replaced by done for. Related: Foregoing; foregone.
Phrase foregone conclusion popularized in "Othello" [III.iii], but Shakespeare's sense was not necessarily the main modern one of "a decision already formed before the case is argued." Othello says it of Cassio's dream, and it is clear from the context that Othello means Cassio actually has been in bed with Desdemona before he allegedly dreamed it.