Merriam-Webster Collegiate Dictionary - behalf

 noun  Etymology: Middle English, from by + half half, side  Date: 14th century interest, benefit; also support, defense argued in his ~ Usage:  A body of opinion favors in with the “interest, benefit” sense of ~ and on with the “support, defense” sense. This distinction has been observed by some writers but overall has never had a sound basis in actual usage. In current British use, on ~ (of) has replaced in ~ (of); both are still used in American English, but the distinction is frequently not observed.