From an Anglo-Norman derivative of jambe (“leg”).
jambeau (plural jambeaux)
- (obsolete) A piece of armour for the leg. [14th-16th c.]
- 1590, Edmund Spenser, The Faerie Queene, II.vi:
- The mortall steele despiteously entayld / Deepe in their flesh, quite through the yron walles, / That a large purple streme adown their giambeux falles.