This appears to be a name or non-standard English word; please provide context for a proper definition.
If this is the name 'Jeff' or 'Jef,' it's a shortened form of 'Jeffrey,' which comes from Germanic roots meaning 'peace' and 'spear.' Cannot provide etymology for non-standard usage.
Names are windows into history—Geoffrey became Jeff, which shows how formal names evolve into casual nicknames in speaking, which eventually became written.
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