A "shameless plug" is a colloquial expression used to describe a situation where someone promotes or advertises something, often themselves or their own work or achievements, in a very self-promotional or unabashed manner, without feeling embarrassed or apologetic about it. In other words, it's when someone shamelessly promotes their own interests or agenda without any pretense of modesty or humility.

For example, if someone says, "I wrote a fantastic book, and you should all buy it right now," they are making a shameless plug for their book because they are promoting it openly and without reservation. It's called "shameless" because the person is not trying to downplay their self-promotion or be discreet about it; they are being very bold and unapologetic in their promotion.