a colloquial expression “ниже плинтуса”

This expression is used to emphasize a very low level of something. It may mean somebody’s bad mood or somebody’s disrespect of another person.

The literal meaning is “below the plinth”.

Some examples of what can be “ниже плинтуса”:
somebody’s mood;
somebody’s authority or status;
quality of goods or service;
effectiveness;
an evaluation.

There is also an expression, “опустить ниже плинтуса” (“to drop below the plinth”). This means to criticise somebody or something severely, to completely drop their rating.