• noun (pl. wallies) Brit. informal a silly or inept person.
— ORIGIN perhaps a shortened form of the given name Walter: the use possibly arose from an
incident at a 1960s pop festival when a Wally became separated from his companions, his name being
taken up as a chant by the crowd following numerous loudspeaker announcements.
If you're looking for a fairly inoffensive insult then wally is your man, so to speak! It's heyday was the 1980's where it was used prolifically in soaps and sit-coms (such as 'Only Fools and Horses')
As always with Brit Slang, Wally has an alternative meaning ... it is also an Estuary/London name for a pickled cucumber or gherkin!