Acronym for "Life Gets Complicated Sometimes"; Typically used as a poor (yet universally accepted) excuse for malicious, boorish, unfeeling, and generally unacceptable behavior.
Sorry for kicking your sister in the face after she dropped the cheerios. LGCS, man." ; "Sorry for telling everyone you're a herpes-ridden demon slut. LGCS." ; "Sorry for running away from home, Mom. LGCS.