Lapse's in concentration that usually occur in the afternoon - typically 3.30pm. These lapse's in concentration typically occur because you are hungry.
It is a very common and popular expression in New Zealand and Australia.

The Continental Cup brand (soup) has produced many ads showing the consequences of three thirtyistis
I have three thirtyitis so I am taking a break (from work)
von charlene_nz 22. Oktober 2009
The lack of concentration experienced during mid-day.
boss: JOOOOHHNNSOOONN, where is that report i asked for..
johnson: huh.. what.. where am i...

A classic example of threethirtyitis
von coinroller 19. August 2007

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