Mencken say US whippletree is UK splinter-bar.
I'll bet not many members know what a whippletree is.
Webster's 1913 Dictionary

Definition: \Whip"ple*tree`\, n. [See {Whip}, and cf.
1. The pivoted or swinging bar to which the traces, or tugs,
of a harness are fastened, and by which a carriage, a
plow, or other implement or vehicle, is drawn; a
whiffletree; a swingletree; a singletree. See

[People] cut their own whippletree in the woodlot.

2. (Bot.) The cornel tree. --Chaucer.

"The old gray mare, she pfft on the whippletree,
pfft on the whippletree, many long years ago."

I wonder what a cornel tree is.

Webster's 1913 Dictionary

Definition: \Cor"nel\ (-n?l), n. [OF. cornille, cornoille, F.
cornouille, cornel berry, LL. cornolium cornel tree, fr. L.
cornus, fr. cornu horn, in allusion to the hardness of the
wood. See {Horn}.]
1. (Bot.) The cornelian cherry ({Cornus Mas}), a European
shrub with clusters of small, greenish flowers, followed
by very acid but edible drupes resembling cherries.

2. Any species of the genus {Cornus}, as {C. florida}, the
flowering cornel; {C. stolonifera}, the osier cornel; {C.
Canadensis}, the dwarf cornel, or bunchberry.