fraternize

Both fraternize and fraternal (meaning "of, relating to, or involving brothers") come to us, by way of Medieval Latin, from Latin frater, meaning "brother." Other frater descendants in English include friar, fraternity, and confraternity ("a society devoted especially to a religious or charitable cause"). Even brother itself shares a relationship with frater. These days, although fraternize can still refer to a brotherly association or simple friendliness, it often occurs in contexts, such as fraternizing with the enemy, implying friendliness toward someone who would be better avoided.