Not so much disused but rarely used these days:

• Nincompoop. (Fun fact: it’s a mangling of Latin “non compos mentis” = “not of sound mind”)

• Gibbering wretch.

• Navel starer.

• Halfwit.

• Dunce

Actually rare / disused:

• Flibbertygibbet. Means “airhead” which itself is a kind of weird word, but is better known. Both usually applied to girls and young women, but can also be used for males, etc.