An Australian colloquialism; describing someone intellectually impaired or moronic.
In the case of this phrase, the term "roo" is an abbreviation for kangaroo, an Australian marsupial.

Used emphatically, the phrase is sometimes amended to "a few roos loose in the top paddock".

To have a roo loose in the top paddock:

After John spent all those years on weed, he talked liked he had a roo loose in the top paddock.
by MrKapper July 10, 2006
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