A postillion is the 'cut card' that protects the bottom card in the pack in games such as poker.
The standard definition of that word is:
a person who rides the leading nearside (left-hand side) horse of a team or pair drawing a coach or carriage, especially when there is no coachman.
And I read at wictionary this etymology:
From Middle French postillon, and its likely source, Italian postiglione (“guide for driver of post-coach”), from posta (“post”).
How and when did the cut-card become known as a postillion?