TTWD stands for "This Thing We Do" and is often used by the bdsm/kink community to describe non-vanilla practices or activities they may engage in, such as spanking, flogging, bondage, roleplay, power exchange etc.
"This Thing We Do" (TTWD) and "Our Thing" were
originally used by the Mafia beginning
in the early 1900s when they would not use the word Mafia, La Cosa Nostra etc. It has also been adopted by
secret societies and then later by the bdsm/kink community. It was meant to be overly general to keep a true identity secret.