маймуна = monkey
"маймуница" is the written gibberish you are looking for:)

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Note added at 17 mins (2007-06-01 11:47:57 GMT)
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On a second thought, "маймунки" is better. Please ignore my previous post, it should definitely be "маймунки" (maimounki).

These comments come only to support the proposal of bsb_2 - who, I hope, will register his answer, as it is the correct one. This is why I have set my confidence level to "lowest".
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The question refers to the term used for normally written text in Cyrillic letters which appears (in display or print) in other characters due to wrong encoding.

The oldest and most widespread term for this is "маймуни" [may'muni] (= monkeys), most commonly encountered in the phrase "(излиза) на маймуни" = (comes out) in gibberish.

In Bulgarian, the word "маймуна" (= monkey) and its derivatives are often used to denote something distorted, usually in a funny way, but without much sense. An example is the phrase "правя се на маймуна" (= play the monkey) which means intentionally and grotesquely distorting one's behaviour/posture/face expression without a serious reason.

Back in MS-DOS times "маймуни" meant the Latin accented chatacters and graphic symbols from the second half (128-255) of the code table, when appearing instead of the Cyrillic characters that should reside there in Cyrillized fonts. Now, with the variety of possible wrong encoding transformations (ANSI - KOI - Unicode), the word means any wrongly encoded characters which render the text unreadable.

It should be noted, however, that this usage of "маймуни" is colloquial. It belongs to the jokingly called 'Bulgarian Computer English' - an informal lingo used by computer users and pros alike, which does not pretend (and does not care, actually) to be 'officialized'.

As bsb_2 noted, a more 'official' term to replace "маймуни" would be "йероглифи" [yero'glifi] (= hierogplyphs) - not meaning the actual Far-Eastern, Egyptian etc. characters but in the figurative meaning of something barely intelligible. BTW it is used also to characterize illegible handwriting.

"Маймунки" [may'munki] is a diminutive form meaning "little monkeys". IMO it is much more rarely used in this context than "маймуни". Same applies to "маймуняци" [may'munyatsi] (= male monkeys) - especially given that the typical word form in Bulgarian is "маймуна" (the female animal).
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As for "маймуница" [may'munitsa], it has a different meaning. It denotes the **intentionally** used 'transliteration alphabet' when writing in Bulgarian using Latin letters (plus eventually some numbers) by phonetic similarity.

Therefore, it is an intelligible way of writing, not the incomprehensible result of a hardware/software glitch, malfunction or wrong settings.

Etymology:
In Bulgarian, the Latin alphabet is called "латиница" [la'tinitsa], and the Cyrillic one - "кирилица" ['kirilitsa]. Analogously (same word ending) comes the newly coined word "маймуница", meaning "monkey alphabet" or "monkey writing".

Note that this does not imply any disrespect to the Latin alphabet; it is rather a reflection of the public opposition in Bulgaria to the 'creeping Latinization' of the Bulgarian language. Nowadays, when using Latin letters for writing Bulgarian text on a computer is no longer a technically determined necessity but a matter of choice, doing it is considered poor taste, and is generally frowned upon.

There are also other names like "мейлица" ['meylitsa], "чатица" ['chatitsa] etc. originating from "mail" and "chat" respectively, but they are much less common.
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"Методиевица" [me'todievitsa] or, as some prefer, "методица" [me'toditsa] is a term originally proposed by the president of the Bulgarian Internet Society. Supporters argue that using Latin letters in virtual communication is a natural development towards a digraphic existence of the Bulgarian language, should be accepted as such, and through the name tribute should be paid to St. Methodius - who, together with his brother St. Cyril, created the Cyrillic alphabet.
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Not to lose the point after this lengthy explanation:

The correct term is "маймуни" (= monkeys) with a typical usage "излиза на маймуни" (= comes out in monkeys). If the text is of a more official nature, it would be more adequate to use "йероглифи" (= hieroglyphs).

HTH