pony up

The story

Originally often poney. US early 19th century. According to Michael Quinion, probably not from the monetary sense of pony and instead from Latin "Legem pone mihi Domine viam iustificationum tuarum" (“Teach me, O Lord, the way of thy statutes”), a line from Psalm 119, verses of which are traditionally used in prayer in Anglican churches on the 25th of the month, becoming associated particularly with March 25th, a traditional day of debt settlement. This etymology has been accepted by the American Heritage Dictionary.