wakarimasen

Very useful word for kyuuryou dorobou in order to get out of work and doings things when they don't want to be bothered. It is tough finding new ways to slack off or look busy at work, but when confronted with doing actual work, use this phrase as much as possible. When asked anything, I simply reply with

"wakarimasen"

"I don't know"

It works!

BONUS!!! Try saying it in a few different ways depending on situation, hell, even geographical location:

"wakarimasen" - formal, so not used much often in casual speech. I say it to my bucho.
"wakaranai" - Tokyo dialect, informal
"wakarahen" - Kansai dialect
"wakarimahen" - nise kansaiben
"wakannai" - when you are lazy and don't want make the extra effort to pronounce all syllables
"wakarane" - rude male speech, especially young ones. Also used by ko gals
"wakanne" - same as above "wakarane", but with no effort for full pronunciation
"wakaran" - again rude male speech with no effort to pronounce

Who said that Japanese was easy? 8 ways to say conjugate the same verb all dependent on situation, place, etc...

But my utmost favorite way to say it, which shows my countryside Kanagawa/Machida roots is...

"wakarane be"

Along with "I don't know", these words can also mean "I don't understand".

Japanese for "I don't know" or "I don't understand". The standard polite negative conjugation of the verb wakaru, "to understand".

Nonnative speakers are often confused between wakaru and a verb of similar meaning: shiru, "to know". One might be tempted to use shirimasen instead of wakarimasen, especially when asked a question to which one doesn't know the answer. However, in Japanese, shiru tends to have the connotation "I don't know, and I don't care". (You might think of it as similar to replying, "As if I would know.") Hence it may come across as odd--such as if someone asked you where you are--or downright rude.

One exception is when you speak of knowing a tangible thing, especially a person. In that case, one would use shiru and it would sound strange to use wakaru. (Thank you eliserh for reminding me.)