Generate a random number between 1 and 64, then use that number to print the corresponding line from the /usr/share/dict/words file.
echo $(awk -v min=1 -v max=64 'BEGIN{srand(); print int(min+rand()*(max-min+1))}') | tr -d ' ' | xargs -I{} sed -n "{}p" /usr/share/dict/words