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)
by charlene_nz October 22, 2009
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