The story
From weak + minded. Probably related to weak (“mentally or intellectually deficient”).
First known use, by meaning
- from 1530s Having or exhibiting a mind that lacks steadfastness, resoluteness or especially judgment; not strong-minded.
- from ca. 1700 Mentally or intellectually deficient; feeble-minded; half-witted.
Etymology from Wiktionary, CC BY-SA 4.0.