"Glasgow smile" and "sardonic smile" explained

First, the Glasgow smile:

A Glasgow smile (also known as a Chelsea smile, or a Glasgow, Chelsea, or Cheshire grin) is a wound caused by making small cuts on the corners of a victim's mouth, then beating or stabbing him or her until the muscles in the face contract, causing the cuts to extend up the cheeks to the victim's ears. This leaves a scar in the shape of a smile, hence the name. The act is usually performed with a utility knife or a piece of broken glass, leaving a scar which causes the victim to appear to be smiling broadly.. The practice is said to have originated in Glasgow, Scotland,[citation needed] but became popular with English street gangs (especially among the Chelsea Headhunters, a London-based hooligan firm, where it is known as a "Chelsea grin" or "Chelsea smile")...

The Glasgow smile has been inflicted on characters in multiple films and television programs, including Green Street, House of Tolerance, The Krays, Sons of Anarchy, Pan's Labyrinth, and 2008's The Dark Knight, in which Heath Ledger as The Joker both has the scar and carves it on numerous victims. 

Then, the more traditional "sardonic" one:

A sardonic action is one that is "disdainfully or skeptically humorous" or "derisively mocking." Also, when referring to laughter or a smile, it is "bitter, scornful, mocking." Hence, when referring to a person or a personal attribute, it is "[c]haracterized by or exhibiting bitterness, scorn or mockery."

The etymology of sardonicism as both a word and concept is uncertain, but it appears to stem from the name for the Mediterranean island of Sardinia. The Byzantine Greek Suda traces its earliest roots to the notion of grinning (Greek: sairō) in the face of danger, or curling one's lips back at evil...

Risus sardonicus is an apparent smile on the face of those who are convulsing because of tetanus, or strychnine poisoning... This facial expression has also been observed among patients with tetanus. Risus sardonicus causes a patient's eyebrows to rise, eyes to bulge, and mouth to retract dramatically, resulting in what has been described as an evil-looking grin...

In 2009 scientists at the University of Eastern Piedmont in Italy claimed to have identified hemlock water dropwort (Oenanthe crocata) as the plant responsible for producing the sardonic grin. This plant is the candidate for the "sardonic herb," which was a neurotoxic plant used for the ritual killing of elderly people in pre-Roman Sardinia. When these people were unable to support themselves, they were intoxicated with this herb and then dropped from a high rock or beaten to death...

First I've heard of the dropwort.  You learn something every day.