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To take on the appearance of a rubber jonny, after a particular bad event has taken place. e.g. physical exercise, night out
Large people tend towards this description more than the slight, due to having more areas of the body that can go flacid and limp
Condition lasts an indeterminate amount of time, depending on extent of bad event.

Sometimes used to refer to someone who is useless.
'you really have caught jonnyitus after that late night out. you won't be able to do anything for the rest of the day.'

'he has a serious case of jonnyitus'
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