Umpteen is used for a relatively large amount or number. We use it when we want to show that there are very many / a lot of something when we don't know the exact number. It is a casual / informal word that is used in spoken English.
"I've met him umpteen times and I still can't remember his name."
"She's given me umpteen reasons why she couldn't do her homework."
"For the umpteenth time, don't put your feet on the table!" - as an adjective.
Link: Word of the day 'spick and span'
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