"Fuck," comes Barrow's growled response, punctuated by a rattling of the bedframe as he shudders again, "fuck!"

It's not very articulate, but it at least communicates how he feels about being told to display willpower: this is what displaying willpower looks like, when every inch of him wants nothing more than to sling Strange around like a rag doll. He keeps himself turned away, grounded by his grip on the infirmary bed.