expression, ichidan verb
to do something spiteful as one leaves; to take a parting shot
Idiomatic expression literally meaning 'to kick sand with one's hind legs'. Used when someone does something mean or ungrateful right before departing, like a final act of spite.
彼は辞める時に、後足で砂をかけるようなことを言った。
When he quit, he said something spiteful as a parting shot.
恩を仇で返すなんて、まさに後足で砂をかける行為だ。
Repaying kindness with ingratitude is exactly the act of kicking sand as you leave.