mare

An ancient torture device involving a wooden horse, typically used for military punishments. I was so terrified of what my parents would do when they found out I'd failed my exam that I had visions of them making me ride the wooden mare.

A difficult, complicated, or confusing situation. The tax laws in this country are a mare's nest that nobody fully understands.

To walk. "Shanks" refers to one's legs. The store is close enough that we don't need to drive, we can just ride shanks' mare.

Fig. by foot; by walking. (Shank refers to the shank of the leg.) My car isn't working, so I'll have to travel by shank's mare. I'm sore because I've been getting around by shank's mare.

Fig. travel on foot. You'll find that shank's mare is the quickest way to get across town. Is there a bus, or do I have to use shank's mare?

a very confused situation The law on restrictive trade is a mare's nest that scarcely anyone can comprehend.

walking as a method of travel I missed the last bus and had to get home on Shanks's pony.

The passage of time. A folk song attributed to Stephen Foster and supposedly referring to a 19th-century harness-racing horse named Lady Suffolk begins, “Oh, the old gray mare, she ain't what she used to be . . . Many long years ago.” Unkind people used the image to refer women “of a certain age” (or older), although when used by themselves about themselves, it has an air of self-deprecating resignation. For example, a middle-aged woman who leaves the dance floor short of breath after a vigorous jitterbug may wipe her brow, reach for a cold drink, and exclaim, “The old gray mare ain't what she used to be.”

Walking. “Shank” is another word for shinbone. By extension, its use in the phase refers to our legs. “Mare” here is equine transport, and when we walk, we “ride” on shank's mare.