Know why cattywampus was trending last week?
"one who rides," Old English ridere "rider, trooper, knight, mounted warrior," agent noun from ride (v.). Meaning "clause tacked on to a document after first draft" is from 1660s. Related: Riderless.
A provision, usually controversial and unlikely to pass on its own merits, that is attached to a popular bill in the hopes that it will “ride” to passage on the back of the popular bill.