Green’s Dictionary of Slang

(orig. US milit.) to befriend.

1948		N. Mailer Naked and Dead 253: You start buddy-buddying with them and they get knocked off.
1960		(con. 1940s) G. Morrill Dark Sea Running 12: Don’t buddy-buddy with the navy gunners.
1977		L. Heinemann Close Quarters (1987) 81: They pawed the guns and buddy-buddied with us, with their hands and arms around our shoulders.