The proverb ‘great minds think alike’ has a straightforward literal meaning. However, the usage is often ironic, that is, it is used when two unexceptional minds have the same thought.

‘Great minds think alike’ isn’t especially old as proverbs go, but the thought behind it dates from at least the early 17th century. The impressively named Dabridgcourt Belchier wrote this in Hans Beer-Pot, 1618:

Though he made that verse, Those words were made before. Good wits doe jumpe.