From 1968 US college slang, probably related to feeb.
The 1980s backronym Dim-Witted Eastern-Educated Boor derives from apparent social and attitudinal differences between West Coast and East Coast US, and describes a stereotypical Ivy League graduate from Harvard, Yale, etc.
Etymology tree
Anglo-Norman feblebor.
Middle English feble
English feeble
English mind
English -ed
English minded
English feeble-minded
English feebinflu.
English dweeb