It is a... comical approximation of:

どういたしまして

Dō-i-ta-shi-ma-shi-te

Roughly:

Dough Ee Tah She Ma She Tay

... but all said rapidly, together.

It means "You're welcome", but is maybe slightly more formal than it is casual.

The joke is that it is maybe what a native English speaker would hear, when a native Japanese speaker says "You're welcome" in Japanese.

... It does not hardly make any sense in text alone, it makes a lot more sense if its actually spoken aloud.

The reverse of this kind of thing... is how a bunch of English terms /phrases have been oddly/poorly translated or transliterated into 'Engrish'.

Most Japanese people I have met have a very good sense of humor about this kind of thing, they think its funny that, without a lot of practice speaking English, they suck at speaking English, and vice versa, native English speakers with no practice speaking Japanese, suck at speaking Japanese.

Like, uh, 'Engrish' itself as a term... is a thing, because in Japanese, they do not have such a distinct difference between 'L' and 'R'.

They use a sound that is roughly in the middle, in between L and R, they usually never learn or use the two as distinct sounds, if they grow up speaking only Japanese.

(Though this could be changing somewhat due to modern internet culture / communications?)

So... they often struggle to learn these two distinct phonemes, sort of how a native English speaker would struggle to learn maybe some of the phonemes in other languages, that either are not present or are very rarely used in English.

You tend to learn phonemes, the building blocks of words, distinct mouth sounds... you learn them best when you are young, its much more difficult to get your brain and mouth to learn new phonemes when you are older.