Abarim Publications' online Biblical Hebrew Dictionary
The curious root-verb כסל (kasal) probably originally meant something like to be thick or plump. It evolved simultaneously towards a positive flavor with the meaning to be confident, and a decidedly negative with the meaning to be stupid (possibly through being gullible or naive).
But on the other hand, perhaps this verb isn't split in two meanings, but states rather clearly that confidence is no virtue and equals stupidity. In the Bible this verb occurs only once, in Jeremiah 10:8. Its derivations are:
- The masculine noun כסל (kesel), meaning loins (Job 15:27, Leviticus 3:4), stupidity (Ecclesiastes 7:25, Psalm 49:13), or confidence (Job 31:24, Proverbs 3:26).
- The feminine noun כסלה (kisla), meaning stupidity (Psalm 85:8), or confidence (Job 4:6).
- The masculine noun כסיל (kesil), meaning fool or dullard (Psalm 49:10, Proverbs 1:22).
- The identical masculine word כסיל (kesil), meaning either the constellations in general, or more specific the constellation Orion. Perhaps as a response to the mythology of the surrounding nations, Egypt for instance, the Hebrew word for Orion (Chesil) is exactly the same as the Hebrew word for fool.
- The feminine noun כסילות (kesilut), meaning stupidity (Proverbs 9:13 only).
Note that stellar constellations exist only in man's imagination, and are entirely equal to faces we see in clouds, conspiracies we see in unrelated events, or pseudo-scientific and religious theories we derive from otherwise explicable observations. Equating astrology's most signature stellar constellation with the word for fool clearly reveals the very nature of the means with which the Hebrews acquired and maintained their reality model.