Given a category and a set of five words, find the word from the set that does not belong (i.e. is the least relevant) with the other words in the category. Words are separated by commas.

Category: moral characteristic 
Words: frank, stern, bun, concise, precise
bun

Category: measurement unit 
Words: meter, inch, foot, heel, pound
heel

Category: nutritional value 
Words: skinny, calcium, fat, salt, heavy

skinny