12/31 7:21 AM PT -- A rep for Coach K tells us he's absolutely perplexed as to why he's included in the lawsuit. We're told he had absolutely no knowledge of the sale and wasn't involved in any capacity, nor does he know who Peter Marco is -- the rep says K rarely even buys jewelry for himself -- other than the occasional vintage watch from a seller in Atlanta.
Offset got the drip from a popular jeweler in L.A. but hasn't squared his tab with the guy in over a year ... according to a new suit.
Peter Marco of Extraordinary Jewels Beverly Hills is going after the Migos rapper for $47,000, for what he claims are unpaid fees from an October 2018 transaction at his store.
The legal docs -- obtained by TMZ -- include a copy of an invoice allegedly showing Offset bought some bling for $110k, but only paid $63k at the time ... leaving the balance of $47k.
According to Marco ... his legal team sent Offset a letter in early December demanding the payment within 7 days, but it looks like they didn't get it. So now, the jeweler wants the full amount ... plus interest and attorney fees.
The lawsuit also lists Quality Control Music COO Kevin "Coach K" Lee. The record label reps Migos.
We've reached out to Offset and Coach K ... no word back so far.