Green’s Dictionary of Slang

not be able to speak a threepenny bit v.

[? image of threepenny bit as an insignificant sum]

to be speechless, struck dumb.

[UK]J. Ware Passing Eng. of the Victorian Era 94/2: I couldn’t speak a threepenny bit, but I made myself a luvverley cup o’ tea, an’ I were soon better.