To part thoroughly; to separate effectually. The marriage service in the ancient prayer-books had “till death us depart,” or “till alimony or death us departs,” a sentence which has been corrupted into “till death us do part.”
Before they settle hands and hearts, Till alimony or death departs.
Butler: Hudibras, iii. 3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Depart from Fact Monster: