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cast off=get rid of somebody or something: to reject or abandon somebody or something regarded as no longer useful or attractive
"I cast off that old coat years ago".
"I always had to wear my sister's cast-offs as a child".
You use 'cast off' to describe leaving on a boat eg 'We cast off into the sunset'
I think its related to the removal of the rope holding the boat to the dock.
It can also be used to describe completing a piece of knitting, 'casting off' the stitches
cast off 
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