Jennifer Fessler Denies Sleeping with West Wilson, Threatens Ciara Miller with Legal Action

'Real Housewives' star Jennifer Fessler is not messing around when it comes to Ciara Miller's claim that she slept with West Wilson ... implying she's ready to sue Miller.

Here's the deal ... on Friday, Miller took to Threads to respond to Fessler calling Wilson a "golden retriever" and adding he didn't mean any harm when he started shacking up with Amanda Batula -- he just wants to have a good time.

While fans expressed outrage at Fessler's comments, Miller claimed she was only saying it because "they slept together too" -- a serious accusations given Fessler's been married for more than 25 years.

Fans ran with the rumor ... but Wilson and Fessler are calling BS on it. Wilson claimed the allegation was news to him -- while Fessler responded in a social media post by calling the claim "categorically untrue and defamatory."

She added if this rumor were true, she'd have no recourse ... but, since it's not, "this can get more complicated" -- seemingly threatening to get lawyers involved.

Fessler adds she hopes the two sides can rectify this situation ASAP before it goes any further.

Obviously, Ciara's still pissed at her ex for hooking up with her longtime pal ... even after the "Summer House" cast ripped Batula to shreds at the reunion which filmed last month.

Fessler doesn't want to get caught up in the drama though ... and it looks like she's willing to use the courts to stay out.