Sentence:  I went over and asked him to let down the <WSD>clews</WSD> or corners of the mainsail, which had been drawn up in order to lessen the useless flapping of the sail against the rigging. 
Meaning of the word marked with <WSD> tags:  The lower corner(s) of a sail to which a sheet is attached for trimming the sail (adjusting its position relative to the wind); the metal loop or cringle in the corner of the sail, to which the sheet is attached.  The trailing corner relative to the wind direction.