The expression “to foot the bill” dates back to a period when women had no means of financial support.
Back then, families offered dowries to entice eligible men to marry their daughters.
The cost of the wedding and the dowry were “footed up,” meaning itemized, then totaled at the bottom of the ledger.
In the fifteenth century, the “foot” was the bottom line, so to foot the bill meant to pay the full amount at the bottom of the invoice.
We say they’ll “foot the bill” when someone is paying all the costs.