Dated slang term for a close male friend, often used among young men. The 公 suffix adds a rough, familiar, or slightly derogatory tone. Comparable to 'dude' or 'bro' in English.
See also: ダチ
あいつは俺のダチ公だ。
That guy's my buddy.
ダチ公とつるんで遊びに行った。
I hung out with my pals and went out.
Slang compound of ダチ (a reversal of たち from 友達) and the familiar/derogatory suffix 公.