The former "Dance Moms" star parked her ass on a comfy chair for a 3-hour mani/ pedi session Thursday in L.A. just days after she was slapped with a 366-day prison sentence. She was convicted on 20 counts of fraud for trying to hide $755k in earnings from the gov.

She seemed unfazed though ... Abby -- who will report to a federal facility near L.A. in about a month and a half -- also gave her future inmate pals a heads-up about her boozing etiquette.

Warning to Abby: the warden frowns upon that kinda thing.