"C'mon Lad! You're all wishbone and no backbone!" Meaning more effort required.
My school rugby master used to run along the line shouting abuse at the sorry assortment of spotty, skinny, snot-nosed kids that made up the junior team;
"Hamilton! you bloody big daisy! Hit that man,hit that man!!"
"You're as much use as a rubber ladder!"
"About as useful as half a pair of scissors!"