E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Plain as a pikestaff. Quite obvious and unmistakable. The pikestaff was the staff carried by pilgrims, which plainly and somewhat ostentatiously announced their devotion. It has been suggested that pikestaff is a corruption of pack-staff, meaning the staff on which a pedlar carries his pack, but there is no need for the change.