Why the word slavery do not appear in the Constitution?

Answer 1

A reasonable guess is that because slavery was already controversial, the portions of the constitution dealing with slaves and the slave trade used the euphemistic terms "other persons" (Article I, Section 2, Clause 3) or "such persons" (Article I, Section 9). Article 4, Section 2, Clause 3 also make oblique reference to slavery by requiring:

No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.

"held to Service or Labour" was a reference to slavery, although it could also have been used to enforce indentured servitude.

James Madison supposed inserted these semantic changes to avoid directly stating that people could be property. The anti-slavery representatives were mollified by not formally accepting that people could be property. Pro-slavery were mollified by the indirect acceptance of slavery.