Scheduled or planned for a particular time or purpose; designated for something.
From the practice of writing schedules on slate boards, from Old French 'esclate' meaning 'splinter'. The figurative sense of 'scheduled' developed in American English in the 19th century.
The phrase 'clean slate' shares the same origin—both expressions preserve the memory of erasable slate boards used in schools and businesses before paper became cheap, when important information was literally written in chalk on slate.
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