Several articles devoted to the 70th wedding anniversary of the Queen and Prince Philip yesterday reminded readers of the vow the Prince made at the Coronation, when he pledged to become his wife's "liege man of life and limb". The original definition of liege man, according to the OED, was, in feudal law "A vassal sworn to the service and support of his superior lord, who in return was obliged to afford him protection, etc". By the 19th century the meaning had widened to "One who serves as though sworn to do so, a faithful follower or subject". Liege entered English from Old French, but the OED says that the ultimate origin is disputed. It seems likely that it is related to the German ledig, meaning 'free' (modern German, 'unmarried').
An obsolete variant of life and limb is lith and limb, where lith means limb, member or joint.