"Fool"

[John Reece]

Also was wondering whether or not calling a brother "you fool," as in the Sermon on the Mount, could also be construed as calling your brother "idiot."

Yes (cp. ιδιwτης (idiōtēs) with αφρονες and ανοητοι, above, in your preferred lexica).

To construe the usage of the biblical word ιδιwτης (idiōtēs) to have the same meaning as calling your brother "idiot" in the modern sense would be anachronistic ― given the fact that the modern usage of the word idiot is not the same as the biblical usage of the word ἰδιώτης (idiōtēs) in the Bible.

Similarly, there is at least a nuance of difference between the modern meaning and usage of the word "idiot" on the one hand, and on the other hand the meaning and usage of the word ῥακά (rhaka) in Matthew 5:22 (see exegesis by France on page 2, post 11 of this thread).

From BDAG:

ἰδιώτης

1. a person who is relatively unskilled or inexperienced in some activity or field of knowledge, layperson, amateur in contrast to an expert or specialist of any kind (the uncrowned person in contrast to the king...).

2. one who is not knowledgeable about some particular group’s experience, one not in the know, outsider. In 1 Cor 14:23f ἰδιῶται and ἄπιστοι together form a contrast to the Christian congregation. The ἰ. are neither similar to the ἄπιστοι (against Ltzm., Hdb. ad loc.), nor are they full-fledged Christians, but stand betw. the two groups, prob. as prospects for membership and are therefore relatively outsiders (ἰδιώτης as a t.t. of religious life e.g. OGI 90, 52 [196 BC], SIG 1013, 6; mystery ins fr. Andania [92 BC]: SIG 736, 16–19 αἱ μὲν ἰδιώτιες . . . αἱ δὲ ἱεραί. In relig. associations the term is used for nonmembers who may participate in the sacrifices: FPoland, Gesch. des griech. Vereinswesens 1909, 247*; 422.—Cp. also Cratin. Iun. Com. [IV BC] fgm. 7 vol. II 291 K. of the Pythagoreans: ἔθος ἐστὶν αὐτοῖς, ἂν τιν᾿ ἰδιώτην ποθὲν λάβωσιν εἰσελθόντα κτλ.). The closer relation which they, in contrast to the ἄπιστοι, held w. the Christian group (so as early as Severian of Gabala [died after 409 AD]: KStaab, Pauluskommentare aus. d. griech. Kirche ’33, p. xxxv; 268) is clearly shown by the fact that they had a special place in the room where the Christians assembled 1 Cor 14:16 (PTomson, Paul and the Jewish Law [CRINT III/1] ’90, 142–44; ἀναπληρόω 4).—DELG s.v. ἴδιος. M-M. TW. Spicq. Sv.

Contrast the above with the following entry from Merriam-Webster:

idiot

1. usually offensive : a person affected with extreme mental retardation
2. a foolish or stupid person

[Just Some Dude]

I'm probably going to have to buy whatever commentaries you tend to use. Thanks.

[John Reece]

I'm probably going to have to buy whatever commentaries you tend to use. Thanks.

[Pat Ferguson]

Pat Ferguson alias Heterodoxus?

That'd be me

I see you're still using this forum like a pedantic podium. Good show!

Perhaps I'll investigate getting my own Tweb forum someday