Ⅰ.
back pay/rent/tax, etc.
► pay, rent, tax, etc. that should have been paid or was expected at an earlier time: »
Most of the back taxes due were for the years 2006 through 2008.
Main Entry: ↑back
Ⅱ.
back tax UK US noun [C or U] TAX
► tax that has not been paid for previous financial years: »
He owes more than $200,000 in back taxes.
»The company is struggling to pay off its enormous back tax bill.
Financial and business terms. 2012.