Sentence:  When placing <WSD>nuts</WSD>, always look for constrictions within the crack, behind which the <WSD>nut</WSD> can be wedged. 
Meaning of the word marked with <WSD> tags:  A shaped piece of metal, threaded by a wire loop, which is jammed in a crack in the rockface and used to protect a climb. (Originally, machine nuts [sense #2] were used for this purpose.)