"We can do nothing for him," said the Tin Woodman sadly. "He is much too heavy to lift. We must leave him here to sleep on forever, and perhaps he will dream that he has found courage at last."
"I'm sorry," said the Scarecrow. "The Lion was a very good comrade for one so cowardly. But let us go on."
The wonderful Wizard of Oz
My take is that "for one so cowardly" here means because the lion is so cowardly, but I have no confidence. Is "for one so cowardly" short for something? Is it a normal expression?