He wasn’t the brightest bulb in the basket

This is another in the collection of what I call “Idiom Overloads”.  It is a mashup of “not the brightest bulb in the pack (or chandelier)” (slow-witted or dull person) and I think “don’t put all your eggs in one basket” (don’t focus all your attention on one thing).  The speaker was probably imagining a bulb in his mind but eggs also look a little like bulbs.  Since many of the idioms describing dull or stupid people have an “in the (blank)” part of the phrase he then added “in the basket.”  “Basket case” (emotionally unstable person) also may be in the mix.

The various sayings describing a dimwit are similar sounding and involve something in a unit or package that is unique, hence the confusion.   I call this phenomenon “idiom overload”.  I have posted other variations on this theme – see https://malaphors.com/2015/12/07/youre-not-the-brightest-toolbox-in-the-shed/ and https://malaphors.com/2013/06/24/not-the-brightest-tool-in-the-shed/.

Also see https://malaphors.com/2016/03/04/hes-not-the-sharpest-light-bulb-in-the-pack/

Another example of idiom overload is describing the obvious: “is the Pope Catholic?”. “Does a bear shit in the woods?” etc.  These get mixed up regularly.   A tip of the hat to Josh Berry for hearing this one and sending it on!