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Self-defecating would mean, literally, shitting on oneself.

Self-depracating would be to deface onself, or to put oneself down before others.

--- to defecate, to deprecate, from Compact Oxford English Dictionary ---

defecate /deffikayt/ • verb discharge faeces from the body.

— DERIVATIVES defecation noun defecatory adjective.

— ORIGIN Latin defaecare, from faex ‘dregs’.

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deprecate /deprikayt/ • verb 1 express disapproval of. 2 another term for DEPRECIATE (in sense 2).

— DERIVATIVES deprecation noun deprecatory adjective.

— ORIGIN originally in the sense ‘pray to ward off evil’: from Latin deprecari, from precari ‘pray’.

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self-deprecating • adjective modest about or critical of oneself.

— DERIVATIVES self-deprecation noun self-deprecatory adjective.

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