Mid 19th century; earliest use found in Proceedings of the Royal Society of London. Apparently from oxamic + -ate (although the adjective is first attested slightly later).
Definition of oxamate in US English:
oxamate
nounˈɒksəmeɪt
Chemistry
A salt or ester, or the anion, of oxamic acid.
Origin
Mid 19th century; earliest use found in Proceedings of the Royal Society of London. Apparently from oxamic + -ate (although the adjective is first attested slightly later).