Daft Wullie
Not the most intelligent of Pictsies, Daft Wullie is nonetheless one of Chalk Hill Clan's best thieves, although every mission he been in charge of has been a failure. Often making suggestions like "Can we swim there?" after another has said it's too far to walk, he hasn't many good ideas, but is always trying to contribute, and 'show willin.' He was in charge of the raiding party that needed turpentine, sheep's wool and Jolly Sailor tobacco. He is a brother of Rob Anybody, who often tells Wullie when he has said something foolish. On one occasion, Rob went a little too far in belittling Daft Wullie's intelligence: and when Wullie protested, Rob actually apologised - by saying that contrary to his previous statement, Daft Wullie does have the brains of a beetle, "an' I'll fight any scunner who says different!" In Wintersmith, Wullie adopts Horace the Cheese, the sentient Lancre Blue cheese that was previously something like a pet of Tiffany's.
Daft Wullie: annotation
In George MacDonald Fraser's autobiographical stories about his time as an officer in the Gordon Highlanders (ie, real-life Feegles in Highland army uniforms) there is a character called "Daft Bob" by his fellows. Bob is a Glaswegian, but so amazingly soft in the heid that he fervently supports neither Celtic nor Rangers but the lowly Glasgow side Partick Thistle. (the parallel is coming from Liverpool and wearing the tangerine scarf of Tranmere Rovers, or residing in Manchester and spurning United or City for Stockport County). Bob also correctly divines that his brand-new commanding officer has, in his first three days with the Regiment, fallen foul of the byzantine rules of the Officers' Mess and ended up on a charge. As well as being daft enough to notice what should not be his concern, he then sets McDonald-Fraser right with well-meant good advice, a thing a private soldier should not normally do to his officer. Hence "Daft Bob" - for either not knowing or not especially caring what the rules are, he just is, and regularly proceeds to say what he's thinking, however daft or ill-advised it is. It is clear that Bob is a khaki idiot savant, a trait he shares with the Feegle Daft Wullie.
There is also a "Wee Wullie", a giant of a man and when sober, a good soldier, but when drunk, capable of such acts as declaring war on the entire United States Marine Corps - and winning. In thirty years' service he has managed promotion to the rank of Private.
Rob Anybody's alter-ego in the Highlanders is most probably the dapper barrack-room lawyer Private Fletcher, who serves much the same function for Daft Bob as Rob does to Daft Wullie. (Book: The General Danced At Dawn, Pan paperbacks)
The whole exchange between Daft Wullie and Rob Anybody, over whether Wullie does or does not have the brains of a beetle, is a long-term running gag that Pratchett himself has used at least once before - in an argument between Esme Weatherwax and Gytha Ogg over whether the latter does or doesn't have the morals of a cat. Similar arguments have been used in political debate as well - most recently in the British House of Commons, Labour MP Dennis Skinner describing half the Conservative Party as crooks, and when called to apologise, he said "I withdraw my statement. Half of the Conservative Party are not crooks". Skinner himself would have known of, and was providing his own variation on, a similar story which is attributed (unproven) to, among others, Benjamin Disraeli: describing half of the Cabinet as asses, and when asked to apologise, stating that half of the Cabinet are not asses. (It is not clear whether the party he was referring to was his opponents, or indeed his own.)