Words in the English language can be constructed, it is part of a living language. The construction of words can come from knowing prefixes, root words and suffixes and combining them in a way that allows for communication of the information.
Diety can mean characterized by or inclined to be a particular selection of food.
I am a deity and the diety I may eat at times isn't my favorite choice of foods I would want to eat.
Diet [noun] means a particular selection of food, especially as designed or prescribed to improve a person's physical condition or to prevent or treat a disease:
Y at the end of a word means:
a native English suffix of adjectives meaning “characterized by or inclined to” the substance or action of the word or stem to which the suffix is attached: juicy; grouchy; rumbly; dreamy. Sometimes used to mean “allowing, fostering, or bringing about” the specified action: sippy.