What's indecent about a nut?
A seed is defined as a "plant structure (as a spore or small dry fruit) capable of producing a new plant." So a seed is anything that can produce a new plant.
A nut is defined partially as a "a hard-shelled dry fruit (as a peanut in the shell) or seed (as a Brazil nut) with an inner kernel." (There's more to the definition, but that's the important part.)
From reading both definitions, you could conclude that nuts are seeds that have hard outer shells. So a nut always contains a seed, but a seed isn't necessarily a nut.