'stealthily' originates from the Old English word 'stælth,' where 'stælth' meant 'theft' or 'secret action.'

'stælth' transformed into the Middle English word 'stelthe,' and eventually became the modern English word 'stealth,' from which 'stealthily' is derived.

Initially, it meant 'secret action or theft,' but over time it evolved into its current meaning of 'in a secretive manner.'