"Word" as used in WTYM and similar expressions relates back to a declaration of Truth or Truthfulness. It may be derived / appropriated from standard English as a shortening of "I give my word (of honor)". Word To Your Mother is an emphasizing phrase, meaning that my words are true and I would swear it to your/my mother, or a reply emphasizing that your words are true and I agree with you. Overall a derivation or corruption of Word To The Mother, popularized in Vanilla Ice's rap to "Ice, Ice Baby".
You got that right! Word To Your Mother!
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