In the introduction to his Cambridge edition of Terence’s Eunuchus, John Barnsby compares Terence to his forebear Plautus. He mentions in passing that Plautus had employed seven different Latin words for ‘stupid’ in a single line, Bacchides 1088. This line reads
stulti, stolidi, fatui, fungi, bardi, blenni, buccones and is a metrical tour de force. I thought it would be interesting to look these up in Michiel de Vaan’s Etymological Dictionary of Latin and the other Italic Languages.
Vaan notes that the first two words, stultus and stolidis have been derived from the same PIE root *stel ‘to place’, with the shift of meaning ‘standing’ > ‘inert’ > ‘insensible; stupid’.
Latin fatuus is ascribed a complex etymology by which Fatuus, an alternative name for the oracular god Faunus, has come to be used pejoratively as ‘silly’. (Cf. how English genius can be used as a term of abuse.)
Latin fungi and blennus are not found in Vaan’s dictionary at all, but their etymologies are straightforward. Lewis & Short’s dictionary lists the former under fungus ‘mushroom’, so it is supposedly an extension of that. Latin blennus is a loan from Greek βλεννός ‘idiot’.
For bardus Vaan notes that two earlier commentators have assumed a loanword, possibly from Etruscan. Finally, He considers buccō under bucca ‘puffed, filled out cheek, mouth’, which has often been considered a loanword from Celtic.