つまらないって英語でなんて言うの？

Bore to tears or bore stiff or bore the pants off. Weary someone through extremely dull talk or uninteresting action.
Tedious = monotonous, dull, deadly dull, uninteresting, unexciting, unvaried
Mind-numbing = so extreme or intense as to prevent normal thought.
to watch paint dry = this is used to refer to an activity that you consider extremely boring

This is not my cup of tea is a mild way to express your boredom or dislike for something at the same time indicating you appreciate others may like it or find it interesting.
This is so uninteresting, is a stronger response and clearly indicates you dont like the speech and are bored by it.
Excruciatingly painful means it is boring to an intensely painful degree. Excruciating it is used to emphasize how embarrassing, awkward, or tedious/boring something is, like a very very boring speech.

That was boring
Meaning = that wasn’t very interesting at all. I was bored

I didn't think was very interesting
Meaning = I wasn’t interested in that. I didn’t catch my attention

What a dull speech that was
Meaning = dull means something that was boring.

You can also say :
Well that was a drag
Meaning= that was boring / felt like it took ages to be over

Well that dragged on a bit
Meaning = that went on for too long, so I was bored

That could have been much more interesting
Meaning = that was boring
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Dull is a way of saying something is boring, not interesting or not fun.
“This party is so dull”

Monotonous is something that does not have variety. Usually something very repetitive and not enjoyable.
“Washing the dishes is very monotonous”

Tedious means the same as the words above but also something that takes too long, is slow and unexciting. This is usually used when talking about a repetitive task.
“Learning how to write kanji is very tedious”

1. Boring - Not interesting, tedious

   2. Uninteresting - Not interesting.
      Example:-
      A. I fell asleep in class because the lecture was BORING.

   B. The speaker was so UNINTERESTING that I left the event.

Tiresome - causing one to feel bored or annoyed

Dry speech- form of expression that the speech had no form anything that was interesting and it was not lively.

When something is not interesting, you can use a number of phrases to describe it. You may use the words: uninteresting, boring, monotonous, dull.

You may use the above words in the following ways:
-The principal's speech was so long and uninteresting, I started to doze off.
-Watching cricket is so boring. It plays all day and there is hardly anything interesting that usually happens.
-My life is so dull/monotonous. There is nothing interesting that ever happens.

The two most common words to describe something that is not interesting are "uninteresting" and "boring." If you call something boring, it can be seen as rude so it might be better to use uninteresting in a professional setting. I hope that this helps. :)

When talking about something that is not interesting we can add the -un prefix to the word to make it, "uninteresting," which means the opposite.
In addition, we can also just use the adjective, "boring," which basically has the same meaning.

Example sentences :
- That speech was very boring.
- This subject is very uninteresting.

"Boring!" is a standard word to describe an uninteresting person/situation. As in, A: "Did you watch the football last night?"B: "No I don't watch the footy, I find it really boring!".

"Mundane!" is another word to describe something uninteresting, tedious, boring. As in, A: "How was your day today?" B: "Just another mundane Monday".

"Stick a needle/pins in my eye(s)" is an idiom to describe something really, really boring. "The presentation at assembly today made me want to stick pins in my eyes!" or A: "Do you want to come to glee club?" B: "No offence, but I'd rather stick a needle in my eye!".

"Boring" is a common adjective that is used by native speakers use to describe something that is not interesting.
EX) The meeting was very boring. It lasted for 3 hours and I did not talk once.

You can also describe the same situation as "a drag." This saying literally describes something that is not interesting and decreases your energy ("drags" your energy down).
EX) The meeting was a drag.