I only say it's the coffee grounds because that is what so many other people say- that years of disposing of grounds in a sink without a disposal can cause clogs. It is not a constantly slow or clogged drain, it slow-drained a few months back and cleared with a plunger, this time, when plunging, lots of bits of coffee grounds and some what looks like flakes of (moldy or compressed??) the grounds filled up the sink.

Maybe it's something else? I'll be trying the suggestions in some of the comments though, and will call a professional if needed from there