Between hay and grass. Neither one thing nor yet another; a
hobbledehoy, neither a man nor yet a boy.
Between cup and lip.
Between Scylla and Charybdis. Between two equal dangers; on the horns
of a dilemma. (See Charybdis.) Between two fires. Between
two dangers. In war, an army fired upon from opposite sides is in
Between two stools you come to the ground:
“Like a man on double business bound, I stand in pause where I
shall first begin, and both neglect.” “He who hunts two hares leaves one
and loses the other.” Simul sorbere ac flare non possum. The
allusion is to a children's game called “The Ambassador,” also a
practical joke at one time played at sea when the ship crossed the
line. Two stools are set side by side, but somewhat apart, and a cloth
is covered over them. A person sits on each stool to keep the cloth
taut, and the ambassador is invited to sit in the middle; but, as soon
as he is seated, the two rise and the ambassador comes to the ground.
Between you and me
(French, entrenous). In confidence be it spoken. Sometimes, Between you and me and the gate-post. These phrases, for the most
part, indicate that some ill-natured remark or slander is about to be
made of a third person, but occasionally they refer to some offer or
private affair. “Between ourselves” is another form of the same phrase.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894