What does brethren mean?

Answer 1

Biblically an specifically the Old Testament the word "Brother" and "Neighbour" means member of own (Hebrew) tribe.

Example;

Leviticus, Chapter 19, verses 16 to 18:

16. Thou shalt not go up and down as a talebearer among thy people ; neither shalt thou stand against the blood of thy neighbour: I am the Lord.

17. Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke thy neighbour, and not suffer sin upon him.

18. Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighour as thyself: I am the Lord.

Deuteronomy 23:19. Thou shalt not lend upon usury to thy brother; usury of money, usury of victuals, usury of any thing that is lent upon usury:

20. Unto a stranger thou mayest lend upon usury; but unto thy brother thou shalt not lend upon usury: that the Lord thy God may bless thee in all that thou settest thine hand to in the land whither thou goest to possess it.

Answer 2

Brethren is the plural form of "brother." This is an old English word/term and is not typically used in modern day. Brethren can also be used to describe more of a "brotherhood," such as in Christian organizations or fraternities, rather than actual blood brothers.

Answer 3

"Thy brethren" is old English for "Your brothers"