What is self-defecate?

Answer 1

It means to release fecal matter (excrement) on oneself.

It is possible the asker meant to ask about self-deprecation. That would mean to put yourself down or make light of yourself. A small amount of that is healthy. It is healthy to admit to one's own faults to the point they can laugh about them. On the other hand, it is not healthy to be overly critical of oneself. To admit to faults and laugh about them is a sign of high self-esteem, while running from faults or obsessing over them is a sign of low self-esteem. If you can laugh at yourself, you really won't care whether others laugh at you or not.

Answer 2

"Self-defecate" is not a recognized term. It appears to be a misspelling or misinterpretation of related terms like self-deprecate or self-defecate, which respectively mean to belittle oneself or to eliminate waste from the body.