Taken from a "dave" person who used to say hiya to all and everyone in rather a shrill voice - the term then rolled round in that everyone would then say "hiya dave" to him mimicking his shrill voice
hiya or "hiya dave" then also came to be used to mark an act of
outstanding stupidity or incompitence - followed by the sticking of
the tongue in the
lower lip (like in the spacker call)
Used in the similar vein as "der" would be used