In Behalf of Your "On Behalf Of"

I just heard someone say "in behalf of" and it didn't make any sense. Isn't is supposed to be "on behalf of"?

Well, this one sent me to my special lingua hero, Bryan A Garner and his Dictionary of Modern American Usage. There is a difference. You didn't include the sentence that threw you for a loop, so I can't help you there; but, according to Garner, the expression, "in behalf of," means "in the interest of or for the benefit of." Frankly, I've rarely heard the expression and believe we're more likely to say "in the interest of" or simply "for."

An example would be:

The teacher worked overtime in behalf of her students' academic advancement.

The teacher worked overtime in the interest of [or for] her students' academic advancement.

"On behalf of," writes Garner, means "as the agent or representative of."

Lawyers usually speak on behalf of their clients; district attorneys usually speak on behalf of the State.