Green’s Dictionary of Slang

1. (Irish) anything of small value.

1530		J. Imrie et al. Burgh Court Bk. Selkirk (1960) 108: Jhone [...] said to Dounaldsone that the hors suld nocht be the vor ane plauk, for ve haif na haist in our rydding .
1689		Irish Hudibras in Bliss Irish Writings from the Age of Swift (1979) 126: What though of ready ne’er a plack / Yet many a plugg of good toback / It cost me to come to dis port .
1791		‘Brother Rook’ Willy Wood & Greedy Grizzle 5: To prove me but a stupid ass, / For buckling thus [...] / A romping girl not worth a plack.
1802		R. Anderson ‘Will & Kate’ Cumberland Ballads (1805) 18: They pick’d my pocket i’ the thrang, / And de’il a plack had I.
1812		J. Bell Jr. (ed.) Rhymes of Northern Bards 6: Tho’ be not worth a plack is / His own coat on his back.
1824		J. Hogg Justified Sinner 93: The saving of [my life] would not have cost you a plack, yet you refused to do it.

2. (Ulster) a mouthful.

1804

J. Orr ‘To the Potatoe’ in Poems on Various Pubjects 36: Wi’ them galore, an’ whyles a plack / To make me frisky, / I’ll fen an’ barley freely lack.