adjective
rotten
RAH-tun/ˈrɑːtn/
Deteriorated through decay.
The rotten pear has collapsed brown patches and mold across its surface.

Etymology
Old Norse rotinn, “decayed,” is a close relative of the Scandinavian source of rotten. Physical decomposition supplied the image for moral corruption and unsound character or quality.
In context
If you leave a rotten apple alone with a lot of good apples, it's not gonna be long before that rot becomes contagious
More examples
- She throws away the rotten apple before it spoils the others.
- A rotten potato gives the pantry a foul smell.
- The fallen tree is rotten inside, though its bark looks intact.
Forms
rotten · rottener · rottenest
rot verb — To break down through natural decay.
rottenness noun — The state of being damaged by decay.