A term used to hide the fact that you are actually crying due to being moved emotionally by implying that you are crying because someone must be cutting onions.
When Jacksaw the video of the lost dog being reunited with its owner he began to tear up and exclaimed, "Who's cutting onions!"
A slang term for crying while observing something tearjerking, such as a sad film scene. This refers to both a ninja's alleged stealth abilities, and the irritation of the eyes by the fluids in an onion.