The word has a birthday. The Federal Reserve wrote it into Regulation Z in 1969, and that year it was accurate.
A teller looked at your check, saw you were short, and covered it. Ad hoc, the government called it. Individuals, one check at a time. Favors do not carry an annual percentage rate, so the Board excepted this one from the definition of a finance charge. Then the teller went away. Software sets your limit now, before you walk into the store, and never tells you the number.
The exception never moved. It is fifty-seven years old, and it is the only thing standing between thirty-five dollars and a figure the bank would have to print.