I can't buckle to. I can't give my mind to work. The allusion is to buckling on one's armour or belt.
“Throth, it wouldn't lave a laugh in you to see the parson dancin' down the road on his way home, and the ministher and methodist praicher cuttin' the buckle as they went along.” —W. B. Yeats: Fairy Tales of the Irish Peasantry, p. 98 (see also p. 196).
“I took a girl to dinner who talked buckle to me.” —Vera, 154.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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