Definition:

Something that is arcane is mysterious, obscure, and difficult to understand—usually because it's known only by a small, specialized group.

Example:

-       "Quantum physics always sounded fascinating, but the math behind it is just too arcane for me."

-       "Some people collect spells from fantasy novels. I collect arcane Latin grammar rules that no living soul actually uses."

Definition:

Originally from boxing, this idiom means someone (or something) is so worn out, defeated, or incapacitated that they’re temporarily or permanently out of action. It’s often used humorously for people or things that are “out of commission.

Example:

-       "After staying up all night with the baby, I was totally down for the count by noon and fell asleep on the couch mid-sentence."

-       "Basil's down for the count. I guess photosynthesis can’t save you from neglect."