noun
(in phrase on (US also in) behalf of or on someone's/something's behalf)
1in the interests of a person, group, or principle:
he campaigned on behalf of the wrongly convicted four
2as a representative of:
he had to attend the funeral on Mama’s behalf
3on the part of; done by:
this wasn’t simply a philanthropic gesture on his behalf
Origin:
Middle English: from a mixture of the earlier phrases on his halve and bihalve him, both meaning 'on his side' (see by, half)