outside
Inside is the opposite word of outside
Inside is the opposite word of outside
Inside.
Endo (inside of)
inversion
opposite front side
garden?
Extremity, torso, inside, middle...
For opposite day, just wear what you normally do, but wear it either backwards or inside out. (of course you could do both!)  socks you must wear inside out.
The opposite in location for a gravy or sauce could be a "filling" (inside, not outside).
It is the bend in your arm (on the inside, opposite from your elbow).
No. The idiom, and others like it (e.g. upside down, backside front), goes one way only. The opposite of inside out is right side out.