He will pay you back on the Greek calends.
Greek calends
idiomA time that never occurs; never; when pigs fly.
Origin
disputed originCalque of Latin ad kalendās Graecās; unlike the Roman calendar, the Greek calendar had no calends.
The committee promised to review our proposal, but we know they will get to it on the Greek calends.
The politician pledged to address the systemic inequality, yet his vague timeline suggested the reforms would arrive only on the Greek calends.