supportasse

From Wikipedia:

A supportasse (or underpropper) is a stiffened support for a ruff or collar. Essential items of courtly fashion in the late 16th and early 17th centuries, supportasses are sometimes called piccadills (picadils, pickadills), whisks, or rebatos, terms used at different times for both the supporters and the various lace or linen collar styles to which they were attached.

Not to be confused with ass support.