left: [13] The Old English word for ‘left’ was winestra. Etymologically this meant ‘friendlier’ (it is related to Swedish vän ‘friend’), and its euphemistic application to ‘left’ is a reminder that historically the left-hand side of the body has been superstitiously regarded as of ill omen. To call it ‘friendly’ (a usage which survives in Swedish vänster and Danish venstre ‘left’) was an attempt to placate the evil forces of the left. (Latin sinister ‘left’ is similarly fraught with negative connotations.
It too had euphemistic origins – it came from a source meaning ‘more useful’ – and it developed the figurative senses ‘unfavourable’, ‘injurious’, etc, taken over and extended by English in sinister [15].) An ancestor of left existed in Old English – left or *lyft. But it meant ‘weak’ or ‘foolish’, and it was not until the 13th century that it came to be used as the partner of right.
Its ultimate origins are not known. [left etymology, left origin, 英语词源]