‘Unreasonable" also means what it has been understood to mean in this context for at least half a century. The expression aptly describes conduct which is vexatious, designed to harass the other side rather than advance the resolution of the case, and it makes no difference that the conduct is the product of excessive zeal and not improper motive. But conduct cannot be described as unreasonable simply because it leads in the event to an unsuccessful result or because other more cautious legal representatives would have acted differently. The acid test is whether the conduct permits of a reasonable explanation. If so, the course adopted may be regarded as optimistic and as reflecting on a practitioner's judgment, but it is not unreasonable.’