That is why such day is called “the day of the tinsmith”.

This expression is only used in informal speech. There is a synonym which can also be used both in informal and in polite speech: “не суть важно”.

There are other expressions with similar (but not absolutely the same) meaning:
“неважно”
“не имеет значения”
“не играет роли”
“не главное”

Examples of usage of the expression “дело десятое” from the Internet:
“Мне кажется, что качество техники в вопросе безопасности авиаперевозок – дело десятое.”
“как это работает и почему — дело десятое”
“нравится это кому или нет – дело десятое”
“Повлияло ли именно это на выход из строя жесткого диска или нет – дело десятое”
“замаскирован или не замаскирован, это дело десятое”

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.

“спалить”
To discover somebody’s secret, to disclose somebody’s shady dealings, especially by seeing something what a person(s) is(are) doing. To see the process itself or the immediate result. Sometimes it may mean “to make public”.

The passive form is “спалиться”. This may mean “to accidentally disclose some information about oneself”. This word is also quite common in criminal slang. For example, when somebody is stealing something and fails.

“пропалить”
To have tracked down, to discover, to find out somebody’s dealings, especially by seeing something.

The word “гаджет” means an electronic gadget. As far as I know, it means some electronic device, not a mechanical one.

There is another loanword “трэш” from the English word “thrash”. This word appeared relatively long ago. It is used in the context of heavy music. It is one of numerous branches of heavy metal.

The most stupid examples of adoption are: “дистрибьютор” (distributor, that is “распространитель”), “мерчандайзер” (merchandiser, that is “торговец”), “меседж” or “мессадж” (message, that is “сообщение”, “послание”).

Some more modern, less common and absolutely redundant loanword is “мерчандайз”
(merchandise, assuming the meaning “sale” – “продажа”, “распродажа”). This word is used in the context of entertainment. The most typical example is when a group arrives in a city with their concert and there is some stuff on sale – CDs, t-shirts, etc. Sometimes this sale is called “мерчандайз” in announces.

There are many other less common loadwords in Russian, especially in a professional slang.

There are many loadwords in the lexicon of IT specialists. This adoption of English words is normal because information technologies come from the West. Some of those words
turn into a mix of English root and Russian prefix, suffix and ending. They are interesting and funny, I kind of like some of them. Maybe I will write an article about them later.

There is another poem-like enumeration of verbs that differ from others
in the way they
are declined. This enumeration is long, like a small poem, and I don’t remember that rule and those verbs now.

Here are some verbs and expressions for something that makes an impression:
“охренеть!” or “охренеть можно!” (indicating an impression or a surprise; suitable both for good and for bad)
“я хренею” or “я охреневаю” (for funny, surprising or upsetting things or situations)

Some verbs describing different things:
“хреновый”, “хреново” (bad)
“хреновня”, “хренотень”, “хрень” (stuff, bullshit; it may be a material object or somebody’s words)
“хреновина” (thingy, thingumajig, doohickey, doosenwhacker; no negative view)

“хренов”
damned, darned, bloody
Used to express disapproval, negative emotions, discontent about something or somebody.
Examples:
“Водитель хренов. Ездит как сумасшедший.” (“Damn driver. He drives like he is crazy.”)
“Специалист хренов. Только хуже сделал.” (“Damn specialist. He just have made it worse.”)

The word “хреново” may be used alone.
Example:
- Мы никуда не едем. (“We are not going anywhere”)
- Хреново. (“That’s too bad”)

The word “охрененно” or “охренительно” can mean:
1) Something very good.
“Там просто охренительно!” (“That place is amazing!”)
2) “very”, “extremely”
“Это охрененно дорого” (“This is very expensive”)
There is also an adjective: “охрененный” or “охренительный”, respectively.

The interjection “охренеть!” expresses an impression or the feeling of surprise.
It can be used in good and bad situations, and also for sarcasm.
This interjection is pronounced with different intonation in different contexts.

The word “однохренственно” is not very common but you still can encounter it. It is a derivative from the expression “один хрен” meaning “it makes no dirrefence”.
Example:
“Мне это однохренственно” – “It makes no dirrefence for me”
It may be more likely to encounter something like this: “Да мне как-то однохренственно” or “да мне один хрен” (it emphasizes indifference).

You can use the root “фиг” instead of “хрен” (with many but NOT all of the words mentioned above) to make similar words and expressions with the same meaning. In this case they will be a little less rough. There are also a couple of stronger word roots, which are used in obscene words and expressions.

Let’s recap a couple of popular words: “хреновый” is “bad” and “охренительный” is “good”.

All these expressions are slang and must not be used in polite or formal speech.

I hope I had covered almost all common expressions with the root “хрен”. There are plenty of options to use them.