Answer:
The term "mine eyes smell onions" is commonly used to signify that someone is sobbing or feeling upset. It derives from Shakespeare's play "All's Well That Ends Well," when the character Parolles states, "I say, mine eyes be onions: I shall weep anon." The expression implies that the person's tears are so intense that they can smell the onions claimed to trigger tears. Because when cutting onions you'll immediately feel tear in your eyes the gas is called syn-propanethial-S-oxide, which is released when you cut into the onion. This gas irritates the eyes and stimulates the lachrymal glands, which produce tears. Which is why people say "mine eyes smell onions"