Green’s Dictionary of Slang

1. a bully, a blustering swaggerer.

1896		‘Frederick Benton Williams’ (H.E. Hamblen) On Many Seas 317: Every man aboard, from the captain down, prides himself on being a ‘bucko,’ — a fighter, that is to say.
1898		F.P. Dunne Mr Dooley in Peace and War 208: He was a fine bucko.
1915		Truth (Brisbane) 25 July 12/2: [T]he golden-toothed bucko gave way to a mirthful chuckle [...] that, heard in a sick-room, might, readily be mistaken for the death-rattle.
1916		P. Kyne Cappy Ricks 114: I’d make that bucko suffer.
1930		R.E. Howard ‘Sailors’ Grudge’ Fight Stories Mar. 🌐 I was a fighting man [...] the terror of all first mates and buckos afloat.
[1931		R.E. Howard ‘TNT Punch’ Action Stories Jan. 🌐 A bucko, as you likely know, is a hard-case mate, who punches his crew around].
1954		J.E. Macdonnell Jim Brady 71: You shouldn’t have much to worry about if any of those Grammar buckos try to do you over.
1980		J. McClure Spike Island (1981) 274: This is when the fights start. This is what they’re looking for, these local buckos — so-called supporters, but we don’t recognise them as such.

2. a (spirited young) man.

1879		H. Hartigan Stray Leaves (2nd ser.) 3: [S]ome of the ‘buckos’ were perfectly capable of running a gauntlet [i.e. of military ‘crimes’]extending from pitch and toss to sacrilege!
1915		O.R. Cohen ‘The Wild Man’ All-Story 28 Aug. 🌐 I wasn’t surprised to see the bucko share catching honors with Thomson.
1918		Hall & Niles One Man’s War (1929) 329: Talked to two young buckos from the 95th Squadron.
1922		Joyce Ulysses 306: That’s the bucko that’ll organize her, take my tip.
1934		‘The Killer’ in Lomax & Lomax Amer. Ballads and Folk Songs 403: I’m a ra’rin’, flarin’ bucko, not afraid to play my hand.
1939		‘Flann O’Brien’ At Swim-Two-Birds 227: A game bucko if you like. Be damned but he wouldn’t die!
1952		B. MacMahon Children of the Rainbow 1: Ah, the lovely world of youth and soon enough the buckos would be brought to heel!
1961		(con. 1890–1910) ‘Flann O’Brien’ Hard Life (1962) 70: In God’s name, he said loudly, what is that bucko up to?
1980		J. McClure Spike Island (1981) 17: For although Ireland still has its dashing young ‘buckos’ the word has almost disappeared from everyday use in Liverpool.
1994		J. O’Connor Secret World of the Irish Male (1995) 83: Them buckos look after their own.

3. a general term of address, e.g. my bucko.

1888		A. Trumble Mott Street Poker Club 27: ‘Ah! me bucco, so I have ye at last’.
1892		F.P. Dunne in Schaaf Mr Dooley’s Chicago (1977) 42: Well Frank me bucko, an’ how ar-re ye?
1907		S.E. White Arizona Nights 116: ‘What?’ says I. ‘Elucidate, my bucko.’.
1915		‘Sapper’ Michael Cassidy 162: ‘And phwat of the issue of rum?’ [...] ‘You may think yourself lucky, my bucko, if you ever get another’.
1917		F. Packard Adventures of Jimmie Dale (1918) I v: This ain’t any police job, my bucko.
1929		B. Duffy Rocky Road 202: ‘I have you at last, me bucko!’ said the policeman.
1935		J. Conroy World to Win 67: And why not, I ask ye, me bucko?
1943		‘Myles na gCopaleen’ Faustus Kelly in ‘Flann O’Brien’ Stories & Plays (1973) 124: You’re roight, me bucko.
1956		J. Curtis Look Long Upon a Monkey 38: I understand you, me bucko.
1966		P. Larkin letter 5 Mar. in Thwaite Sel. Letters (1992) 383: Next sign will be the heavy tread of the British law enforcement officer, come along o me, my bucko, we know your sort.
1970		C. Brown Down All the Days 88: You’re bloody Darby-and-Joan now, me bucko.
1989		C. Hiaasen Skin Tight 26: Not together, bucko, trust me.
1993		A. Rodriguez Spidertown (1994) 48: Spider he tol’ me I get my shit at midnight an’ it’s already [...] Twelve-thirty, bucko.
2001		S. King Dreamcatcher 518: Lovely to hear you, bucko!
2006		C. Hiaasen Nature Girl 232: I’ve seen her in action, bucko.
2015		L. McInerney Glorious Heresies 254: Your move, bucko.
2019		S.M. Jones Lives Laid Away [ebook] ‘Hold on, bucko [...] Who the hell are you?’.