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If CNRTL is to be believed
https://www.cnrtl.fr/definition/cotillon#:~:text=Culotte ou ...

it's not so much a petticoat, even it is worn beneath another skirt, Cotte, jupe de dessous , as a particular style of skirt, espèce de jupes plissées derrière et sur les côtés.

Lexis defines cotillon (note the spelling) as jupon porté autrefois par les femmes du peuple, and jupon as Pièce de lingerie, maintenue à la taille par une ceinture ou un caoutchouc, qui soutient l'ampleur de la jupe, indicating the original concept (1319) as tunique à manches.

For my old Larousse, it's Jupe de dessous des paysannes.

Bref, it's not easy to know what we're talking about.

Cotte above and the root of cotillon will indeed have given us 'petticoat', and, my ignorance showing, as a petticoat, I have learnt that a petticoat is more than just the frilly bits that, when I was a boy, got girls into a tizz if it was pointed out to them that it was showing, but also:
- A small coat worn by men beneath the doublet
- A garment [unspecified] worn by women, girls and young children, further specified as
a. A skirt as distinct from a bodice, worn either externally or beneath the gown or frock
b. An underskirt
c. The skirt of a woman's riding habit
d. The rudimentary garment worn by women among primitive or uncivilized peoples [cache-sexe?]

Alain Rey's historical dict. gives its derivation from cotte, i.e. an upper-body garment, and mentions that a variant, cotteron was a petite jupe courte et étroite, especially the cotillon de laine porté par les paysannes de la Flandre. Further, it is said Seul le dérivé COTILLON est encore vivant pour n'être pas resté strictement un terme d'habillement : traditionnellement, il désigne une jupe de dessous portée par les femmes du peuple (encore dans les parlers méridionaux …"

On the Ouèbbhe we find an additional complication: Le cotillon est une sorte de jupon apparu au XVIIe siècle. C’est plus précisément une jupe de dessous, qui se porte au-dessus d’un jupon (et en-dessous d’une jupe vous l’aurez compris). Le terme est surtout associé aux paysannes. Le cotillon est encore aujourd’hui une pièce indispensable du costume provençal.

Taffetas seems to suggest something rather more expensive than peasants might wear, but it is possible, these days at least, that the fabric is used for traditional dress, and if it is in a Provençal setting, it is possible that it is the skirt between the petticoat and the outer dress.

So the fuller context might play a part in the decision.

In uncertainty, I would plump for 'underskirt' (as distinct from a slip or petticoat).

Whatever it is, it should be made compulsory attire in these days of energy economy.



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Note added at 4 hrs (2024-03-28 17:37:51 GMT)
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Ah! The question now is whether the use of, first, jupon, then cotillon, is a matter of the 'elegant variation' [Fowler] the French are so fond of, or are they two different things. If a proponent of said elegant variation, and assuming 'petticoat' has been taken for jupon, 'underskirt' might indeed do the trick.

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Note added at 4 hrs (2024-03-28 17:38:30 GMT)
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