Behalf is the interest of another, in the phrase acting for them. On behalf of, on his behalf, speak on our behalf.
British and American usage split here: on behalf of means as the agent of somebody, and in behalf of means for their benefit, though the second is rare now and mostly American. The word only appears in these phrases. Note the Old English by his side behind it.
1nounbehalfThe interest of another, in the phrase acting for them.
“I'm writing on behalf of my father, who is too ill to write himself.”
behalf is an interest represented.
From Old English bi healfe, by the side of, from be, by, plus healfe, side. Half is the same word.
Old English
bī healfe — by [his/her] side
Middle English
bihalve — nearby
English, today
behalf
Built from
From Middle English bihalve (“nearby”), from Old English bī healfe (“by [his/her] side”), from bī (“by”) + healfe (“side, half”), equivalent to be- + half.
Etymology from Wiktionary, CC BY-SA